Hello, all.
I would like to clarify some bits regarding a possible update for "Xen device tree bindings for the guest" [1].
A bit of context:
We are considering extending "reg" property under the hypervisor node and we would like to avoid breaking backward compatibility.
So far, the "reg" was used to carry a single region for the grant table mapping only and it's size is quite small for the new improvement
we are currently working on.
What we want to do is to extend the current region [reg: 0] and add an extra regions [reg: 1-N] to be used as a safe address space for any
Xen specific mappings. But, we need to be careful about running "new" guests (with the improvement being built-in already) on "old" Xen
which is not aware of the extended regions, so we need the binding to be extended in a backward compatible way. In order to detect whether
we are running on top of the "new" Xen (and it provides us enough space to be used for improvement), we definitely need some sign to
indicate that.
Could you please clarify, how do you expect the binding to be changed in the backward compatible way?
- by adding an extra compatible (as it is a change of the binding technically)
- by just adding new property (xen,***) to indicate that "reg" contains enough space
- other option
The current description is:
- reg: specifies the base physical address and size of a region in
memory where the grant table should be mapped to, using an
HYPERVISOR_memory_op hypercall [...]
Although it says "a region" I think that adding multiple ranges would be
fine and shouldn't break backward compatibility.
In addition, the purpose of the region was described as "where the grant
table should be mapped". In other words, it is a safe address range
where the OS can map Xen special pages.
Your proposal is to extend the region to be bigger to allow the OS to
map more Xen special pages. I think it is a natural extension to the
binding, which should be backward compatible.
Rob, I am not sure what is commonly done in these cases. Maybe we just
need an update to the description of the binding? I am also fine with
adding a new compatible string if needed.
Hi Stefano,
On 06/08/2021 21:15, Stefano Stabellini wrote:
quoted
On Fri, 6 Aug 2021, Oleksandr Tyshchenko wrote:
quoted
Hello, all.
I would like to clarify some bits regarding a possible update for "Xen
device tree bindings for the guest" [1].
A bit of context:
We are considering extending "reg" property under the hypervisor node and
we would like to avoid breaking backward compatibility.
So far, the "reg" was used to carry a single region for the grant table
mapping only and it's size is quite small for the new improvement
we are currently working on.
What we want to do is to extend the current region [reg: 0] and add an
extra regions [reg: 1-N] to be used as a safe address space for any
Xen specific mappings. But, we need to be careful about running "new"
guests (with the improvement being built-in already) on "old" Xen
which is not aware of the extended regions, so we need the binding to be
extended in a backward compatible way. In order to detect whether
we are running on top of the "new" Xen (and it provides us enough space to
be used for improvement), we definitely need some sign to
indicate that.
Could you please clarify, how do you expect the binding to be changed in
the backward compatible way?
- by adding an extra compatible (as it is a change of the binding
technically)
- by just adding new property (xen,***) to indicate that "reg" contains
enough space
- other option
The current description is:
- reg: specifies the base physical address and size of a region in
memory where the grant table should be mapped to, using an
HYPERVISOR_memory_op hypercall [...]
Although it says "a region" I think that adding multiple ranges would be
fine and shouldn't break backward compatibility.
In addition, the purpose of the region was described as "where the grant
table should be mapped". In other words, it is a safe address range
where the OS can map Xen special pages.
Your proposal is to extend the region to be bigger to allow the OS to
map more Xen special pages. I think it is a natural extension to the
binding, which should be backward compatible.
I agree that extending the reg (or even adding a second region) should be fine
for older OS.
quoted
Rob, I am not sure what is commonly done in these cases. Maybe we just
need an update to the description of the binding? I am also fine with
adding a new compatible string if needed.
So the trouble is how a newer Linux version knows that the region is big
enough to deal with all the foreign/grant mapping?
If you run on older Xen, then the region will only be 16MB. This means the
Linux will have to fallback on stealing RAM as it is today.
IOW, XSA-300 will still be a thing. On newer Xen (or toolstack), we ideally
want the OS to not fallback on stealing RAM (and close XSA-300). This is where
we need a way to advertise it.
The question here is whether we want to use a property or a compatible for
this.
I am leaning towards the latter because this is an extension of the bindings.
However, I wasn't entirely whether this was a normal way to do it.
Although I think it would be OK to have a new compatible string, am I
not sure we need it.
In any case, we'll have to be able to recognize and handle the case
where we run out of the space in the provided region. If the region is
too small (16MB) then it just means we'll run out of space immediately
after mapping the grant table. Then, we'll have to use other techniques.
Or perhaps you think that if we had a new compatible string to say "Xen
binding with a larger region" then we could get away with a simpler
implementation in Linux, one that doesn't handle the case where we run
out of space in the region? If that was the case, then I agree that it
would be worthwhile adding a new compatible.
Hi Stefano,
On 06/08/2021 21:15, Stefano Stabellini wrote:
On Fri, 6 Aug 2021, Oleksandr Tyshchenko wrote:
quoted
Hello, all.
I would like to clarify some bits regarding a possible update for "Xen device tree bindings for the guest" [1].
A bit of context:
We are considering extending "reg" property under the hypervisor node and we would like to avoid breaking backward compatibility.
So far, the "reg" was used to carry a single region for the grant table mapping only and it's size is quite small for the new improvement
we are currently working on.
What we want to do is to extend the current region [reg: 0] and add an extra regions [reg: 1-N] to be used as a safe address space for any
Xen specific mappings. But, we need to be careful about running "new" guests (with the improvement being built-in already) on "old" Xen
which is not aware of the extended regions, so we need the binding to be extended in a backward compatible way. In order to detect whether
we are running on top of the "new" Xen (and it provides us enough space to be used for improvement), we definitely need some sign to
indicate that.
Could you please clarify, how do you expect the binding to be changed in the backward compatible way?
- by adding an extra compatible (as it is a change of the binding technically)
- by just adding new property (xen,***) to indicate that "reg" contains enough space
- other option
The current description is:
- reg: specifies the base physical address and size of a region in
memory where the grant table should be mapped to, using an
HYPERVISOR_memory_op hypercall [...]
Although it says "a region" I think that adding multiple ranges would be
fine and shouldn't break backward compatibility.
In addition, the purpose of the region was described as "where the grant
table should be mapped". In other words, it is a safe address range
where the OS can map Xen special pages.
Your proposal is to extend the region to be bigger to allow the OS to
map more Xen special pages. I think it is a natural extension to the
binding, which should be backward compatible.
I agree that extending the reg (or even adding a second region) should be fine for older OS.
Rob, I am not sure what is commonly done in these cases. Maybe we just
need an update to the description of the binding? I am also fine with
adding a new compatible string if needed.
So the trouble is how a newer Linux version knows that the region is big enough to deal with all the foreign/grant mapping?
If you run on older Xen, then the region will only be 16MB. This means the Linux will have to fallback on stealing RAM as it is today.
IOW, XSA-300 will still be a thing. On newer Xen (or toolstack), we ideally want the OS to not fallback on stealing RAM (and close XSA-300). This is where we need a way to advertise it.
The question here is whether we want to use a property or a compatible for this.
I am leaning towards the latter because this is an extension of the bindings. However, I wasn't entirely whether this was a normal way to do it.
Cheers,
--
Julien Grall
Hi Stefano,
On 06/08/2021 21:15, Stefano Stabellini wrote:
quoted
On Fri, 6 Aug 2021, Oleksandr Tyshchenko wrote:
quoted
Hello, all.
I would like to clarify some bits regarding a possible update for "Xen device tree bindings for the guest" [1].
A bit of context:
We are considering extending "reg" property under the hypervisor node and we would like to avoid breaking backward compatibility.
So far, the "reg" was used to carry a single region for the grant table mapping only and it's size is quite small for the new improvement
we are currently working on.
What we want to do is to extend the current region [reg: 0] and add an extra regions [reg: 1-N] to be used as a safe address space for any
Xen specific mappings. But, we need to be careful about running "new" guests (with the improvement being built-in already) on "old" Xen
which is not aware of the extended regions, so we need the binding to be extended in a backward compatible way. In order to detect whether
we are running on top of the "new" Xen (and it provides us enough space to be used for improvement), we definitely need some sign to
indicate that.
Could you please clarify, how do you expect the binding to be changed in the backward compatible way?
- by adding an extra compatible (as it is a change of the binding technically)
- by just adding new property (xen,***) to indicate that "reg" contains enough space
- other option
The current description is:
- reg: specifies the base physical address and size of a region in
memory where the grant table should be mapped to, using an
HYPERVISOR_memory_op hypercall [...]
I actually forgot to reply on this one and only remembered now. There are some funny things happening in Xen on Arm when mapping the grant table. At the moment, we rely on the grant table to always be mapped for all the components (toolstack, OS, firmware...) at the same place.
If the region end up to be re-used by something else, then it will end up to unmap it... We would need to fix it before we can fully re-use the region.
quoted
Although it says "a region" I think that adding multiple ranges would be
fine and shouldn't break backward compatibility.
In addition, the purpose of the region was described as "where the grant
table should be mapped". In other words, it is a safe address range
where the OS can map Xen special pages.
Your proposal is to extend the region to be bigger to allow the OS to
map more Xen special pages. I think it is a natural extension to the
binding, which should be backward compatible.
I agree that extending the reg (or even adding a second region) should be fine for older OS.
quoted
Rob, I am not sure what is commonly done in these cases. Maybe we just
need an update to the description of the binding? I am also fine with
adding a new compatible string if needed.
So the trouble is how a newer Linux version knows that the region is big enough to deal with all the foreign/grant mapping?
If you run on older Xen, then the region will only be 16MB. This means the Linux will have to fallback on stealing RAM as it is today.
IOW, XSA-300 will still be a thing. On newer Xen (or toolstack), we ideally want the OS to not fallback on stealing RAM (and close XSA-300). This is where we need a way to advertise it.
The question here is whether we want to use a property or a compatible for this.
I am leaning towards the latter because this is an extension of the bindings. However, I wasn't entirely whether this was a normal way to do it.
Cheers,
Hi Stefano,
On 06/08/2021 23:26, Stefano Stabellini wrote:
On Fri, 6 Aug 2021, Julien Grall wrote:
quoted
Hi Stefano,
On 06/08/2021 21:15, Stefano Stabellini wrote:
quoted
On Fri, 6 Aug 2021, Oleksandr Tyshchenko wrote:
quoted
Hello, all.
I would like to clarify some bits regarding a possible update for "Xen
device tree bindings for the guest" [1].
A bit of context:
We are considering extending "reg" property under the hypervisor node and
we would like to avoid breaking backward compatibility.
So far, the "reg" was used to carry a single region for the grant table
mapping only and it's size is quite small for the new improvement
we are currently working on.
What we want to do is to extend the current region [reg: 0] and add an
extra regions [reg: 1-N] to be used as a safe address space for any
Xen specific mappings. But, we need to be careful about running "new"
guests (with the improvement being built-in already) on "old" Xen
which is not aware of the extended regions, so we need the binding to be
extended in a backward compatible way. In order to detect whether
we are running on top of the "new" Xen (and it provides us enough space to
be used for improvement), we definitely need some sign to
indicate that.
Could you please clarify, how do you expect the binding to be changed in
the backward compatible way?
- by adding an extra compatible (as it is a change of the binding
technically)
- by just adding new property (xen,***) to indicate that "reg" contains
enough space
- other option
The current description is:
- reg: specifies the base physical address and size of a region in
memory where the grant table should be mapped to, using an
HYPERVISOR_memory_op hypercall [...]
Although it says "a region" I think that adding multiple ranges would be
fine and shouldn't break backward compatibility.
In addition, the purpose of the region was described as "where the grant
table should be mapped". In other words, it is a safe address range
where the OS can map Xen special pages.
Your proposal is to extend the region to be bigger to allow the OS to
map more Xen special pages. I think it is a natural extension to the
binding, which should be backward compatible.
I agree that extending the reg (or even adding a second region) should be fine
for older OS.
quoted
Rob, I am not sure what is commonly done in these cases. Maybe we just
need an update to the description of the binding? I am also fine with
adding a new compatible string if needed.
So the trouble is how a newer Linux version knows that the region is big
enough to deal with all the foreign/grant mapping?
If you run on older Xen, then the region will only be 16MB. This means the
Linux will have to fallback on stealing RAM as it is today.
IOW, XSA-300 will still be a thing. On newer Xen (or toolstack), we ideally
want the OS to not fallback on stealing RAM (and close XSA-300). This is where
we need a way to advertise it.
The question here is whether we want to use a property or a compatible for
this.
I am leaning towards the latter because this is an extension of the bindings.
However, I wasn't entirely whether this was a normal way to do it.
Although I think it would be OK to have a new compatible string, am I
not sure we need it.
Let's assume we don't add a new compatible string, property... How do would you prevent the following two issues?
1) XSA-300: A frontend can DoS the backend
2) Existing Xen expects the grant-table to be mapped at the exact same place.
2# could potentially be solved by reserved the first range for the grant table. For 1#, I think we need a compatible string (or property).
What else do you have in mind?
FAOD, relying on the region to always be big enough would not be an acceptable solution to me :). A frontend may find a new way for a frontend to exhaust the region (*hint* virtio *hint*).
In any case, we'll have to be able to recognize and handle the case
where we run out of the space in the provided region. If the region is
too small (16MB) then it just means we'll run out of space immediately
after mapping the grant table. Then, we'll have to use other techniques.
Right, one of the other techniques is likely to steal RAM page. Which means that a frontend could potentially DoS the backend. This will be a lot easier to trigger with virtio as the DM tends to cache the mappings.
So I think we ought to prevent stealing the RAM if a new kernel is running on a new Xen.
Or perhaps you think that if we had a new compatible string to say "Xen
binding with a larger region" then we could get away with a simpler
implementation in Linux, one that doesn't handle the case where we run
out of space in the region? If that was the case, then I agree that it
would be worthwhile adding a new compatible.
We will need to keep the code to steal RAM for the forseeable future because a newer Linux may run on an older Xen setup. So simplicity is not the reason here.
I have provided the reason above.
Cheers,
--
Julien Grall
Hello, all.
Gentle reminder.
On 07.08.21 01:57, Julien Grall wrote:
Hi Stefano,
On 06/08/2021 23:26, Stefano Stabellini wrote:
quoted
On Fri, 6 Aug 2021, Julien Grall wrote:
quoted
Hi Stefano,
On 06/08/2021 21:15, Stefano Stabellini wrote:
quoted
On Fri, 6 Aug 2021, Oleksandr Tyshchenko wrote:
quoted
Hello, all.
I would like to clarify some bits regarding a possible update for "Xen
device tree bindings for the guest" [1].
A bit of context:
We are considering extending "reg" property under the hypervisor node and
we would like to avoid breaking backward compatibility.
So far, the "reg" was used to carry a single region for the grant table
mapping only and it's size is quite small for the new improvement
we are currently working on.
What we want to do is to extend the current region [reg: 0] and add an
extra regions [reg: 1-N] to be used as a safe address space for any
Xen specific mappings. But, we need to be careful about running "new"
guests (with the improvement being built-in already) on "old" Xen
which is not aware of the extended regions, so we need the binding to be
extended in a backward compatible way. In order to detect whether
we are running on top of the "new" Xen (and it provides us enough space to
be used for improvement), we definitely need some sign to
indicate that.
Could you please clarify, how do you expect the binding to be changed in
the backward compatible way?
- by adding an extra compatible (as it is a change of the binding
technically)
- by just adding new property (xen,***) to indicate that "reg" contains
enough space
- other option
The current description is:
- reg: specifies the base physical address and size of a region in
memory where the grant table should be mapped to, using an
HYPERVISOR_memory_op hypercall [...]
Although it says "a region" I think that adding multiple ranges would be
fine and shouldn't break backward compatibility.
In addition, the purpose of the region was described as "where the grant
table should be mapped". In other words, it is a safe address range
where the OS can map Xen special pages.
Your proposal is to extend the region to be bigger to allow the OS to
map more Xen special pages. I think it is a natural extension to the
binding, which should be backward compatible.
I agree that extending the reg (or even adding a second region) should be fine
for older OS.
quoted
Rob, I am not sure what is commonly done in these cases. Maybe we just
need an update to the description of the binding? I am also fine with
adding a new compatible string if needed.
So the trouble is how a newer Linux version knows that the region is big
enough to deal with all the foreign/grant mapping?
If you run on older Xen, then the region will only be 16MB. This means the
Linux will have to fallback on stealing RAM as it is today.
IOW, XSA-300 will still be a thing. On newer Xen (or toolstack), we ideally
want the OS to not fallback on stealing RAM (and close XSA-300). This is where
we need a way to advertise it.
The question here is whether we want to use a property or a compatible for
this.
I am leaning towards the latter because this is an extension of the bindings.
However, I wasn't entirely whether this was a normal way to do it.
May I please ask for the clarification how to properly advertise that we have extended region? By new compatible or property?
quoted
Although I think it would be OK to have a new compatible string, am I
not sure we need it.
Let's assume we don't add a new compatible string, property... How do would you prevent the following two issues?
1) XSA-300: A frontend can DoS the backend
2) Existing Xen expects the grant-table to be mapped at the exact same place.
2# could potentially be solved by reserved the first range for the grant table. For 1#, I think we need a compatible string (or property).
What else do you have in mind?
FAOD, relying on the region to always be big enough would not be an acceptable solution to me :). A frontend may find a new way for a frontend to exhaust the region (*hint* virtio *hint*).
quoted
In any case, we'll have to be able to recognize and handle the case
where we run out of the space in the provided region. If the region is
too small (16MB) then it just means we'll run out of space immediately
after mapping the grant table. Then, we'll have to use other techniques.
Right, one of the other techniques is likely to steal RAM page. Which means that a frontend could potentially DoS the backend. This will be a lot easier to trigger with virtio as the DM tends to cache the mappings.
So I think we ought to prevent stealing the RAM if a new kernel is running on a new Xen.
quoted
Or perhaps you think that if we had a new compatible string to say "Xen
binding with a larger region" then we could get away with a simpler
implementation in Linux, one that doesn't handle the case where we run
out of space in the region? If that was the case, then I agree that it
would be worthwhile adding a new compatible.
We will need to keep the code to steal RAM for the forseeable future because a newer Linux may run on an older Xen setup. So simplicity is not the reason here.
I have provided the reason above.
Hi Stefano,
On 06/08/2021 23:26, Stefano Stabellini wrote:
quoted
On Fri, 6 Aug 2021, Julien Grall wrote:
quoted
Hi Stefano,
On 06/08/2021 21:15, Stefano Stabellini wrote:
quoted
On Fri, 6 Aug 2021, Oleksandr Tyshchenko wrote:
quoted
Hello, all.
I would like to clarify some bits regarding a possible update for
"Xen
device tree bindings for the guest" [1].
A bit of context:
We are considering extending "reg" property under the hypervisor
node and
we would like to avoid breaking backward compatibility.
So far, the "reg" was used to carry a single region for the grant
table
mapping only and it's size is quite small for the new improvement
we are currently working on.
What we want to do is to extend the current region [reg: 0] and add
an
extra regions [reg: 1-N] to be used as a safe address space for any
Xen specific mappings. But, we need to be careful about running
"new"
guests (with the improvement being built-in already) on "old" Xen
which is not aware of the extended regions, so we need the binding
to be
extended in a backward compatible way. In order to detect whether
we are running on top of the "new" Xen (and it provides us enough
space to
be used for improvement), we definitely need some sign to
indicate that.
Could you please clarify, how do you expect the binding to be
changed in
the backward compatible way?
- by adding an extra compatible (as it is a change of the binding
technically)
- by just adding new property (xen,***) to indicate that "reg"
contains
enough space
- other option
The current description is:
- reg: specifies the base physical address and size of a region in
memory where the grant table should be mapped to, using an
HYPERVISOR_memory_op hypercall [...]
Although it says "a region" I think that adding multiple ranges would
be
fine and shouldn't break backward compatibility.
In addition, the purpose of the region was described as "where the
grant
table should be mapped". In other words, it is a safe address range
where the OS can map Xen special pages.
Your proposal is to extend the region to be bigger to allow the OS to
map more Xen special pages. I think it is a natural extension to the
binding, which should be backward compatible.
I agree that extending the reg (or even adding a second region) should
be fine
for older OS.
quoted
Rob, I am not sure what is commonly done in these cases. Maybe we just
need an update to the description of the binding? I am also fine with
adding a new compatible string if needed.
So the trouble is how a newer Linux version knows that the region is big
enough to deal with all the foreign/grant mapping?
If you run on older Xen, then the region will only be 16MB. This means
the
Linux will have to fallback on stealing RAM as it is today.
IOW, XSA-300 will still be a thing. On newer Xen (or toolstack), we
ideally
want the OS to not fallback on stealing RAM (and close XSA-300). This is
where
we need a way to advertise it.
The question here is whether we want to use a property or a compatible
for
this.
I am leaning towards the latter because this is an extension of the
bindings.
However, I wasn't entirely whether this was a normal way to do it.
May I please ask for the clarification how to properly advertise that we have
extended region? By new compatible or property?
The current compatible string is defined as:
- compatible:
compatible = "xen,xen-<version>", "xen,xen";
where <version> is the version of the Xen ABI of the platform.
On the Xen side it is implemented as:
"xen,xen-"__stringify(XEN_VERSION)"."__stringify(XEN_SUBVERSION)"\0"
So in a way we already have the version in the compatible string but it
is just the Xen version, not the version of the Device Tree binding.
Looking at the way the compatible string is parsed in Linux, I think we
cannot easily change/add a different string format because it would
cause older Linux to stop initializing the Xen subsystem.
So one option is to rely on a check based on the Xen version. Example:
version >= xen,xen-4.16
Or we need to go with a property. This seems safer and more solid. The
property could be as simple as "extended-region":
hypervisor {
compatible = "xen,xen-4.16", "xen,xen";
extended-region;
reg = <0 0xb0000000 0 0x20000 0xc 0x0 0x1 0x0>;
interrupts = <1 15 0xf08>;
};
Julien, do you have a better suggestion for the property name?
On 28.08.21 03:05, Stefano Stabellini wrote:
Hi Stefano
On Fri, 27 Aug 2021, Oleksandr wrote:
quoted
On 07.08.21 01:57, Julien Grall wrote:
quoted
Hi Stefano,
On 06/08/2021 23:26, Stefano Stabellini wrote:
quoted
On Fri, 6 Aug 2021, Julien Grall wrote:
quoted
Hi Stefano,
On 06/08/2021 21:15, Stefano Stabellini wrote:
quoted
On Fri, 6 Aug 2021, Oleksandr Tyshchenko wrote:
quoted
Hello, all.
I would like to clarify some bits regarding a possible update for
"Xen
device tree bindings for the guest" [1].
A bit of context:
We are considering extending "reg" property under the hypervisor
node and
we would like to avoid breaking backward compatibility.
So far, the "reg" was used to carry a single region for the grant
table
mapping only and it's size is quite small for the new improvement
we are currently working on.
What we want to do is to extend the current region [reg: 0] and add
an
extra regions [reg: 1-N] to be used as a safe address space for any
Xen specific mappings. But, we need to be careful about running
"new"
guests (with the improvement being built-in already) on "old" Xen
which is not aware of the extended regions, so we need the binding
to be
extended in a backward compatible way. In order to detect whether
we are running on top of the "new" Xen (and it provides us enough
space to
be used for improvement), we definitely need some sign to
indicate that.
Could you please clarify, how do you expect the binding to be
changed in
the backward compatible way?
- by adding an extra compatible (as it is a change of the binding
technically)
- by just adding new property (xen,***) to indicate that "reg"
contains
enough space
- other option
The current description is:
- reg: specifies the base physical address and size of a region in
memory where the grant table should be mapped to, using an
HYPERVISOR_memory_op hypercall [...]
Although it says "a region" I think that adding multiple ranges would
be
fine and shouldn't break backward compatibility.
In addition, the purpose of the region was described as "where the
grant
table should be mapped". In other words, it is a safe address range
where the OS can map Xen special pages.
Your proposal is to extend the region to be bigger to allow the OS to
map more Xen special pages. I think it is a natural extension to the
binding, which should be backward compatible.
I agree that extending the reg (or even adding a second region) should
be fine
for older OS.
quoted
Rob, I am not sure what is commonly done in these cases. Maybe we just
need an update to the description of the binding? I am also fine with
adding a new compatible string if needed.
So the trouble is how a newer Linux version knows that the region is big
enough to deal with all the foreign/grant mapping?
If you run on older Xen, then the region will only be 16MB. This means
the
Linux will have to fallback on stealing RAM as it is today.
IOW, XSA-300 will still be a thing. On newer Xen (or toolstack), we
ideally
want the OS to not fallback on stealing RAM (and close XSA-300). This is
where
we need a way to advertise it.
The question here is whether we want to use a property or a compatible
for
this.
I am leaning towards the latter because this is an extension of the
bindings.
However, I wasn't entirely whether this was a normal way to do it.
May I please ask for the clarification how to properly advertise that we have
extended region? By new compatible or property?
The current compatible string is defined as:
- compatible:
compatible = "xen,xen-<version>", "xen,xen";
where <version> is the version of the Xen ABI of the platform.
On the Xen side it is implemented as:
"xen,xen-"__stringify(XEN_VERSION)"."__stringify(XEN_SUBVERSION)"\0"
So in a way we already have the version in the compatible string but it
is just the Xen version, not the version of the Device Tree binding.
Looking at the way the compatible string is parsed in Linux, I think we
cannot easily change/add a different string format because it would
cause older Linux to stop initializing the Xen subsystem.
So one option is to rely on a check based on the Xen version. Example:
version >= xen,xen-4.16
Or we need to go with a property. This seems safer and more solid. The
property could be as simple as "extended-region":
hypervisor {
compatible = "xen,xen-4.16", "xen,xen";
extended-region;
reg = <0 0xb0000000 0 0x20000 0xc 0x0 0x1 0x0>;
interrupts = <1 15 0xf08>;
};
Thank you for the detailed analysis, I think, it makes sense.
Julien, do you have a better suggestion for the property name?
Hi Stefano,
On 28/08/2021 01:05, Stefano Stabellini wrote:
On Fri, 27 Aug 2021, Oleksandr wrote:
quoted
On 07.08.21 01:57, Julien Grall wrote:
quoted
Hi Stefano,
On 06/08/2021 23:26, Stefano Stabellini wrote:
quoted
On Fri, 6 Aug 2021, Julien Grall wrote:
quoted
Hi Stefano,
On 06/08/2021 21:15, Stefano Stabellini wrote:
quoted
On Fri, 6 Aug 2021, Oleksandr Tyshchenko wrote:
quoted
Hello, all.
I would like to clarify some bits regarding a possible update for
"Xen
device tree bindings for the guest" [1].
A bit of context:
We are considering extending "reg" property under the hypervisor
node and
we would like to avoid breaking backward compatibility.
So far, the "reg" was used to carry a single region for the grant
table
mapping only and it's size is quite small for the new improvement
we are currently working on.
What we want to do is to extend the current region [reg: 0] and add
an
extra regions [reg: 1-N] to be used as a safe address space for any
Xen specific mappings. But, we need to be careful about running
"new"
guests (with the improvement being built-in already) on "old" Xen
which is not aware of the extended regions, so we need the binding
to be
extended in a backward compatible way. In order to detect whether
we are running on top of the "new" Xen (and it provides us enough
space to
be used for improvement), we definitely need some sign to
indicate that.
Could you please clarify, how do you expect the binding to be
changed in
the backward compatible way?
- by adding an extra compatible (as it is a change of the binding
technically)
- by just adding new property (xen,***) to indicate that "reg"
contains
enough space
- other option
The current description is:
- reg: specifies the base physical address and size of a region in
memory where the grant table should be mapped to, using an
HYPERVISOR_memory_op hypercall [...]
Although it says "a region" I think that adding multiple ranges would
be
fine and shouldn't break backward compatibility.
In addition, the purpose of the region was described as "where the
grant
table should be mapped". In other words, it is a safe address range
where the OS can map Xen special pages.
Your proposal is to extend the region to be bigger to allow the OS to
map more Xen special pages. I think it is a natural extension to the
binding, which should be backward compatible.
I agree that extending the reg (or even adding a second region) should
be fine
for older OS.
quoted
Rob, I am not sure what is commonly done in these cases. Maybe we just
need an update to the description of the binding? I am also fine with
adding a new compatible string if needed.
So the trouble is how a newer Linux version knows that the region is big
enough to deal with all the foreign/grant mapping?
If you run on older Xen, then the region will only be 16MB. This means
the
Linux will have to fallback on stealing RAM as it is today.
IOW, XSA-300 will still be a thing. On newer Xen (or toolstack), we
ideally
want the OS to not fallback on stealing RAM (and close XSA-300). This is
where
we need a way to advertise it.
The question here is whether we want to use a property or a compatible
for
this.
I am leaning towards the latter because this is an extension of the
bindings.
However, I wasn't entirely whether this was a normal way to do it.
May I please ask for the clarification how to properly advertise that we have
extended region? By new compatible or property?
The current compatible string is defined as:
- compatible:
compatible = "xen,xen-<version>", "xen,xen";
where <version> is the version of the Xen ABI of the platform.
On the Xen side it is implemented as:
"xen,xen-"__stringify(XEN_VERSION)"."__stringify(XEN_SUBVERSION)"\0"
So in a way we already have the version in the compatible string but it
is just the Xen version, not the version of the Device Tree binding.
Looking at the way the compatible string is parsed in Linux, I think we
cannot easily change/add a different string format because it would
cause older Linux to stop initializing the Xen subsystem.
AFAICT, Linux doesn't care about extra compatible after "xen,xen". So in theory we could write:
"xen,xen-<version>", "xen,xen", "xen,xen-v2".
So one option is to rely on a check based on the Xen version. Example:
version >= xen,xen-4.16
This option would prevent a stakeholder to backport the work to older Xen.
Or we need to go with a property. This seems safer and more solid. The
property could be as simple as "extended-region":
hypervisor {
compatible = "xen,xen-4.16", "xen,xen";
extended-region;
reg = <0 0xb0000000 0 0x20000 0xc 0x0 0x1 0x0>;
interrupts = <1 15 0xf08>;
};
Julien, do you have a better suggestion for the property name?
I am a bit confused with the name you suggest. To me it looks this would be used to indicate whether "reg" has one or more regions.
But I don't think we need a property for that. What we need to a property for is to indicate whether the first region is safe to use (see the discussion about the grant table).
So can you clarify what you expect to convey with this property? Although, at the moment, I don't have a good name to suggest.
Cheers,
--
Julien Grall
Hi Stefano,
On 06/08/2021 23:26, Stefano Stabellini wrote:
quoted
On Fri, 6 Aug 2021, Julien Grall wrote:
quoted
Hi Stefano,
On 06/08/2021 21:15, Stefano Stabellini wrote:
quoted
On Fri, 6 Aug 2021, Oleksandr Tyshchenko wrote:
quoted
Hello, all.
I would like to clarify some bits regarding a possible update
for
"Xen
device tree bindings for the guest" [1].
A bit of context:
We are considering extending "reg" property under the hypervisor
node and
we would like to avoid breaking backward compatibility.
So far, the "reg" was used to carry a single region for the
grant
table
mapping only and it's size is quite small for the new
improvement
we are currently working on.
What we want to do is to extend the current region [reg: 0] and
add
an
extra regions [reg: 1-N] to be used as a safe address space for
any
Xen specific mappings. But, we need to be careful about running
"new"
guests (with the improvement being built-in already) on "old"
Xen
which is not aware of the extended regions, so we need the
binding
to be
extended in a backward compatible way. In order to detect
whether
we are running on top of the "new" Xen (and it provides us
enough
space to
be used for improvement), we definitely need some sign to
indicate that.
Could you please clarify, how do you expect the binding to be
changed in
the backward compatible way?
- by adding an extra compatible (as it is a change of the
binding
technically)
- by just adding new property (xen,***) to indicate that "reg"
contains
enough space
- other option
The current description is:
- reg: specifies the base physical address and size of a region in
memory where the grant table should be mapped to, using an
HYPERVISOR_memory_op hypercall [...]
Although it says "a region" I think that adding multiple ranges
would
be
fine and shouldn't break backward compatibility.
In addition, the purpose of the region was described as "where the
grant
table should be mapped". In other words, it is a safe address
range
where the OS can map Xen special pages.
Your proposal is to extend the region to be bigger to allow the OS
to
map more Xen special pages. I think it is a natural extension to
the
binding, which should be backward compatible.
I agree that extending the reg (or even adding a second region)
should
be fine
for older OS.
quoted
Rob, I am not sure what is commonly done in these cases. Maybe we
just
need an update to the description of the binding? I am also fine
with
adding a new compatible string if needed.
So the trouble is how a newer Linux version knows that the region is
big
enough to deal with all the foreign/grant mapping?
If you run on older Xen, then the region will only be 16MB. This
means
the
Linux will have to fallback on stealing RAM as it is today.
IOW, XSA-300 will still be a thing. On newer Xen (or toolstack), we
ideally
want the OS to not fallback on stealing RAM (and close XSA-300).
This is
where
we need a way to advertise it.
The question here is whether we want to use a property or a
compatible
for
this.
I am leaning towards the latter because this is an extension of the
bindings.
However, I wasn't entirely whether this was a normal way to do it.
May I please ask for the clarification how to properly advertise that we
have
extended region? By new compatible or property?
The current compatible string is defined as:
- compatible:
compatible = "xen,xen-<version>", "xen,xen";
where <version> is the version of the Xen ABI of the platform.
On the Xen side it is implemented as:
"xen,xen-"__stringify(XEN_VERSION)"."__stringify(XEN_SUBVERSION)"\0"
So in a way we already have the version in the compatible string but it
is just the Xen version, not the version of the Device Tree binding.
Looking at the way the compatible string is parsed in Linux, I think we
cannot easily change/add a different string format because it would
cause older Linux to stop initializing the Xen subsystem.
AFAICT, Linux doesn't care about extra compatible after "xen,xen". So in
theory we could write:
"xen,xen-<version>", "xen,xen", "xen,xen-v2".
Keep in mind that the generic compatible string ("xen,xen") is typically
last. Also we shouldn't rely on their ordering. Considering the definition
of hyper_node:
static __initdata struct {
const char *compat;
const char *prefix;
const char *version;
bool found;
} hyper_node = {"xen,xen", "xen,xen-", NULL, false};
And the following code:
s = of_get_flat_dt_prop(node, "compatible", &len);
if (strlen(hyper_node.prefix) + 3 < len &&
!strncmp(hyper_node.prefix, s, strlen(hyper_node.prefix)))
hyper_node.version = s + strlen(hyper_node.prefix);
It looks like there is potential for breakage. For instance, It looks
like that hyper_node.version could end up being set to "xen,xen-v2"
depending on the success or failure of the first < len check. If not
"xen,xen-v2", I would guess that "xen,xen-version-2" would cause
hyper_node.version to be set to "version-2".
If we were to introduce a new compatible we would need to make it so the
prefix "xen,xen-" does not match it. Something like "xen,api-v2" might
be possible.
quoted
So one option is to rely on a check based on the Xen version. Example:
version >= xen,xen-4.16
This option would prevent a stakeholder to backport the work to older Xen.
quoted
Or we need to go with a property. This seems safer and more solid. The
property could be as simple as "extended-region":
hypervisor {
compatible = "xen,xen-4.16", "xen,xen";
extended-region;
reg = <0 0xb0000000 0 0x20000 0xc 0x0 0x1 0x0>;
interrupts = <1 15 0xf08>;
};
Julien, do you have a better suggestion for the property name?
I am a bit confused with the name you suggest. To me it looks this would be
used to indicate whether "reg" has one or more regions.
But I don't think we need a property for that. What we need to a property for
is to indicate whether the first region is safe to use (see the discussion
about the grant table).
So can you clarify what you expect to convey with this property? Although, at
the moment, I don't have a good name to suggest.
Yeah extended-region is a bad name. It meant to convey that the reg
property will cover a larger (extended) address range.
Hi Stefano,
On 31/08/2021 22:19, Stefano Stabellini wrote:
On Tue, 31 Aug 2021, Julien Grall wrote:
quoted
On 28/08/2021 01:05, Stefano Stabellini wrote:
quoted
On Fri, 27 Aug 2021, Oleksandr wrote:
quoted
On 07.08.21 01:57, Julien Grall wrote:
quoted
Hi Stefano,
On 06/08/2021 23:26, Stefano Stabellini wrote:
quoted
On Fri, 6 Aug 2021, Julien Grall wrote:
quoted
Hi Stefano,
On 06/08/2021 21:15, Stefano Stabellini wrote:
quoted
On Fri, 6 Aug 2021, Oleksandr Tyshchenko wrote:
quoted
Hello, all.
I would like to clarify some bits regarding a possible update
for
"Xen
device tree bindings for the guest" [1].
A bit of context:
We are considering extending "reg" property under the hypervisor
node and
we would like to avoid breaking backward compatibility.
So far, the "reg" was used to carry a single region for the
grant
table
mapping only and it's size is quite small for the new
improvement
we are currently working on.
What we want to do is to extend the current region [reg: 0] and
add
an
extra regions [reg: 1-N] to be used as a safe address space for
any
Xen specific mappings. But, we need to be careful about running
"new"
guests (with the improvement being built-in already) on "old"
Xen
which is not aware of the extended regions, so we need the
binding
to be
extended in a backward compatible way. In order to detect
whether
we are running on top of the "new" Xen (and it provides us
enough
space to
be used for improvement), we definitely need some sign to
indicate that.
Could you please clarify, how do you expect the binding to be
changed in
the backward compatible way?
- by adding an extra compatible (as it is a change of the
binding
technically)
- by just adding new property (xen,***) to indicate that "reg"
contains
enough space
- other option
The current description is:
- reg: specifies the base physical address and size of a region in
memory where the grant table should be mapped to, using an
HYPERVISOR_memory_op hypercall [...]
Although it says "a region" I think that adding multiple ranges
would
be
fine and shouldn't break backward compatibility.
In addition, the purpose of the region was described as "where the
grant
table should be mapped". In other words, it is a safe address
range
where the OS can map Xen special pages.
Your proposal is to extend the region to be bigger to allow the OS
to
map more Xen special pages. I think it is a natural extension to
the
binding, which should be backward compatible.
I agree that extending the reg (or even adding a second region)
should
be fine
for older OS.
quoted
Rob, I am not sure what is commonly done in these cases. Maybe we
just
need an update to the description of the binding? I am also fine
with
adding a new compatible string if needed.
So the trouble is how a newer Linux version knows that the region is
big
enough to deal with all the foreign/grant mapping?
If you run on older Xen, then the region will only be 16MB. This
means
the
Linux will have to fallback on stealing RAM as it is today.
IOW, XSA-300 will still be a thing. On newer Xen (or toolstack), we
ideally
want the OS to not fallback on stealing RAM (and close XSA-300).
This is
where
we need a way to advertise it.
The question here is whether we want to use a property or a
compatible
for
this.
I am leaning towards the latter because this is an extension of the
bindings.
However, I wasn't entirely whether this was a normal way to do it.
May I please ask for the clarification how to properly advertise that we
have
extended region? By new compatible or property?
The current compatible string is defined as:
- compatible:
compatible = "xen,xen-<version>", "xen,xen";
where <version> is the version of the Xen ABI of the platform.
On the Xen side it is implemented as:
"xen,xen-"__stringify(XEN_VERSION)"."__stringify(XEN_SUBVERSION)"\0"
So in a way we already have the version in the compatible string but it
is just the Xen version, not the version of the Device Tree binding.
Looking at the way the compatible string is parsed in Linux, I think we
cannot easily change/add a different string format because it would
cause older Linux to stop initializing the Xen subsystem.
AFAICT, Linux doesn't care about extra compatible after "xen,xen". So in
theory we could write:
"xen,xen-<version>", "xen,xen", "xen,xen-v2".
Keep in mind that the generic compatible string ("xen,xen") is typically
last.
Ok. Even if it is written "xen,xen-<version>", "xen,xen-v2", "xen,xen". I still don't see the problem (see more below).
Also we shouldn't rely on their ordering. Considering the definition
of hyper_node:
static __initdata struct {
const char *compat;
const char *prefix;
const char *version;
bool found;
} hyper_node = {"xen,xen", "xen,xen-", NULL, false};
And the following code:
s = of_get_flat_dt_prop(node, "compatible", &len);
if (strlen(hyper_node.prefix) + 3 < len &&
!strncmp(hyper_node.prefix, s, strlen(hyper_node.prefix)))
hyper_node.version = s + strlen(hyper_node.prefix);
It looks like there is potential for breakage. For instance, It looks
like that hyper_node.version could end up being set to "xen,xen-v2"
depending on the success or failure of the first < len check. If not
"xen,xen-v2", I would guess that "xen,xen-version-2" would cause
hyper_node.version to be set to "version-2".
How so? s points to the beginning of the property. So if the if "xen,xen-4.16" is always first, there there is no way "hyper_node.version" can be set to "version-2".
If we were to introduce a new compatible we would need to make it so the
prefix "xen,xen-" does not match it. Something like "xen,api-v2" might
be possible.
I don't particularly care about the compatible name. This was an example to show that there is no issue with adding an extra compatible. I thought the compatible way was better because at least we don't have to add a new property every single time we extend the bindings.
However, the point of this conversation was to figure out whether the common way to extend the Device-Tree is to add a compatible or a new property. Not what we (Xen Project) prefer.
Cheers,
--
Julien Grall