Re: [PATCH kernel RFC 2/2] vfio-pci-nvlink2: Implement interconnect isolation

On Fri, 22 Mar 2019 14:08:38 +1100
David Gibson [off-list ref] wrote:

On Thu, Mar 21, 2019 at 12:19:34PM -0600, Alex Williamson wrote:

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On Thu, 21 Mar 2019 10:56:00 +1100
David Gibson [off-list ref] wrote:
  

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On Wed, Mar 20, 2019 at 01:09:08PM -0600, Alex Williamson wrote:  

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On Wed, 20 Mar 2019 15:38:24 +1100
David Gibson [off-list ref] wrote:
    

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On Tue, Mar 19, 2019 at 10:36:19AM -0600, Alex Williamson wrote:    

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On Fri, 15 Mar 2019 19:18:35 +1100
Alexey Kardashevskiy [off-list ref] wrote:
      

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The NVIDIA V100 SXM2 GPUs are connected to the CPU via PCIe links and
(on POWER9) NVLinks. In addition to that, GPUs themselves have direct
peer to peer NVLinks in groups of 2 to 4 GPUs. At the moment the POWERNV
platform puts all interconnected GPUs to the same IOMMU group.

However the user may want to pass individual GPUs to the userspace so
in order to do so we need to put them into separate IOMMU groups and
cut off the interconnects.

Thankfully V100 GPUs implement an interface to do by programming link
disabling mask to BAR0 of a GPU. Once a link is disabled in a GPU using
this interface, it cannot be re-enabled until the secondary bus reset is
issued to the GPU.

This defines a reset_done() handler for V100 NVlink2 device which
determines what links need to be disabled. This relies on presence
of the new "ibm,nvlink-peers" device tree property of a GPU telling which
PCI peers it is connected to (which includes NVLink bridges or peer GPUs).

This does not change the existing behaviour and instead adds
a new "isolate_nvlink" kernel parameter to allow such isolation.

The alternative approaches would be:

1. do this in the system firmware (skiboot) but for that we would need
to tell skiboot via an additional OPAL call whether or not we want this
isolation - skiboot is unaware of IOMMU groups.

2. do this in the secondary bus reset handler in the POWERNV platform -
the problem with that is at that point the device is not enabled, i.e.
config space is not restored so we need to enable the device (i.e. MMIO
bit in CMD register + program valid address to BAR0) in order to disable
links and then perhaps undo all this initialization to bring the device
back to the state where pci_try_reset_function() expects it to be.      

The trouble seems to be that this approach only maintains the isolation
exposed by the IOMMU group when vfio-pci is the active driver for the
device.  IOMMU groups can be used by any driver and the IOMMU core is
incorporating groups in various ways.      

I don't think that reasoning is quite right.  An IOMMU group doesn't
necessarily represent devices which *are* isolated, just devices which
*can be* isolated.  There are plenty of instances when we don't need
to isolate devices in different IOMMU groups: passing both groups to
the same guest or userspace VFIO driver for example, or indeed when
both groups are owned by regular host kernel drivers.

In at least some of those cases we also don't want to isolate the
devices when we don't have to, usually for performance reasons.    

I see IOMMU groups as representing the current isolation of the device,
not just the possible isolation.  If there are ways to break down that
isolation then ideally the group would be updated to reflect it.  The
ACS disable patches seem to support this, at boot time we can choose to
disable ACS at certain points in the topology to favor peer-to-peer
performance over isolation.  This is then reflected in the group
composition, because even though ACS *can be* enabled at the given
isolation points, it's intentionally not with this option.  Whether or
not a given user who owns multiple devices needs that isolation is
really beside the point, the user can choose to connect groups via IOMMU
mappings or reconfigure the system to disable ACS and potentially more
direct routing.  The IOMMU groups are still accurately reflecting the
topology and IOMMU based isolation.    

Huh, ok, I think we need to straighten this out.  Thinking of iommu
groups as possible rather than potential isolation was a conscious  

possible ~= potential  

Sorry, I meant "current" not "potential".

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decision on my part when we were first coming up with them.  The
rationale was that that way iommu groups could be static for the
lifetime of boot, with more dynamic isolation state layered on top.

Now, that was based on analogy with PAPR's concept of "Partitionable
Endpoints" which are decided by firmware before boot.  However, I
think it makes sense in other contexts too: if iommu groups represent
current isolation, then we need some other way to advertise possible
isolation - otherwise how will the admin (and/or tools) know how it
can configure the iommu groups.

VFIO already has the container, which represents explicitly a "group
of groups" that we don't care to isolate from each other.  I don't
actually know what other uses of the iommu group infrastructure we
have at present and how they treat them.  

s/that we don't care to isolate/that the user doesn't care to isolate/  

Well, true, I guess we still want to isolate them when possible for
additional safety.  But I don't think we should do so when that
isolation has a real performance cost, which it would in the case of
isolating linked GPUs here.

I don't see that we can presume the user's intentions.  This sounds
like we're putting the kernel into the position of deciding policy.
 

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Though even that is not necessarily accurate, the container represents
a shared IOMMU context, that context doesn't necessarily include
mappings between devices, so the device can still be isolated *from
each other*.  

Well, sure, but it's the only mechanism the user has for indicating
that they don't care about isolation between these device.  That might
be because they simply don't care, but it might also be because they
want to use interlinks between those devices for additional
performance.

Is a user really indicating "I don't care about isolation" by placing
groups within the same container or just taking advantage of a shared
IOMMU context?  Again, we're presuming a policy decision that's never
been defined in the API.  It is possible to have both isolation and
performance, for instance ACS includes a direct translated p2p feature
that allows translated requests a direct path between devices, at least
on the PCIe fabric (where supported by hardware).

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So, if we now have dynamically reconfigurable groups which are a
departure from that design, how can we go from here trying to bring
things back to consistency.  

We don't currently have dynamically reconfigurable groups,  

Ah, ok.  So what were these other use cases you were describing?  Boot
time options which change how the iommu groups are generated?  Or
something else.

        pci=option[,option...]  [PCI] various PCI subsystem options.

                                Some options herein operate on a specific device
                                or a set of devices (<pci_dev>). These are
                                specified in one of the following formats:

                                [<domain>:]<bus>:<dev>.<func>[/<dev>.<func>]*
                                pci:<vendor>:<device>[:<subvendor>:<subdevice>]

        ...
                disable_acs_redir=<pci_dev>[; ...]
                                Specify one or more PCI devices (in the format
                                specified above) separated by semicolons.
                                Each device specified will have the PCI ACS
                                redirect capabilities forced off which will
                                allow P2P traffic between devices through
                                bridges without forcing it upstream. Note:
                                this removes isolation between devices and
                                may put more devices in an IOMMU group.

AIUI, GPU and RDMA were primary use cases for this.  As noted, IOMMU
groups take this into account since it's done at boot time and static,
we're simply disabling isolation switches that would have otherwise
been enabled at points in the topology (specifically enabling p2p as
much as possible), and the IOMMU group grows to match.

The NVLink problem is rather the opposite of this, rather than simply
disabling isolation for a sub-hierarchy and growing the IOMMU group, we
need to enable isolation and then actively, and repeatedly maintain
that configuration as it represents a finer granularity isolation
promise to the system.  And of course we're trying to manage the
isolation of an interconnect for which we have no visibility.

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I think the
question is how do we introduce dynamically configurable groups within
the existing design, because whatever intentions were 7.5 years ago is
rather irrelevant now.  

Well, yeah, I guess.

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VFIO groups are mapped directly to IOMMU groups.  IOMMU groups are the
smallest set of devices which can be considered isolated from other
sets of devices (not potentially, but as configured).  

Hm.. but what actually makes that assumption that this is the case as
configured, rather than just possibly - I'm taking "possible" to mean
possible with this host kernel, not just possible with this hardware.

The IOMMU driver is ultimately responsible for deciding the isolation,
but we have shared code that understands the PCI/e topology.  That code
looks at the actual, current ACS settings to decide the highest point
in the topology where we might have p2p.

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VFIO groups are
the unit of ownership to userspace, therefore a VFIO group must be
(currently) isolated from other groups.  

It must be currently isolated once the VFIO group is instantiated, but
is there actually anything that requires that current isolation before
the iommu group is instantiated as a vfio group.

The iommu-core is incorporating more group-aware support, I believe
default domains are group based and we might have per group reserved
ranges, I'm not sure about that.

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The user owns and manages the
IOMMU context for one or more groups via a container.  A container
allows efficiency in managing a shared IOVA space between groups.  

Yes, and it seems to me an obvious extension to have the container
also permit other efficiencies which aren't compatible with full
isolation.

It might be a control point, yes, but I don't see that we can presume
it.  If we were to automatically enable redirection within a container,
we could wreak havoc on the IOVA space for the user.  ACS also
theoretically provides a hardware path to enable this via direct
translated p2p as well.

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So, if there's a device specific
way to configure the isolation reported in the group, which requires
some sort of active management against things like secondary bus
resets, then I think we need to manage it above the attached endpoint
driver.      

The problem is that above the endpoint driver, we don't actually have
enough information about what should be isolated.  For VFIO we want to
isolate things if they're in different containers, for most regular
host kernel drivers we don't need to isolate at all (although we might
as well when it doesn't have a cost).    

This idea that we only want to isolate things if they're in different
containers is bogus, imo.  There are performance reasons why we might
not want things isolated, but there are also address space reasons why
we do.  If there are direct routes between devices, the user needs to
be aware of the IOVA pollution, if we maintain singleton groups, they
don't.  Granted we don't really account for this well in most
userspaces and fumble through it by luck of the address space layout
and lack of devices really attempting peer to peer access.    

I don't really follow what you're saying here.  

If the lack of isolation between devices includes peer-to-peer channels
then the MMIO of the devices within the group pollutes the IOVA space
of the container.  

Yes, if the permissible IOVA addresses overlap with valid MMIO
addresses.  I don't really see why that's significant, though.  We
already have basically the same situation if there are multiple
devices in a group.  e.g. if you have several devices behind a dumb
PCI-E to PCI bridge, you can't actually prohibit peer-to-peer DMA
between them, and so those devices' MMIOs pollute the IOVA space for
each other, even if they don't for other devices in the container.
There's only so far we can go to prevent the user from shooting
themselves in the foot.

So it's therefore OK to intentionally pollute the container IOVA
space?  I believe we'd like to think that hardware is headed towards
being more isolation-aware and will evolve to the point where we have
typically singleton group.  Perhaps we'll also evolve to supporting ACS
direct translated p2p so that we have both a pristine IOVA space and
direct routes between devices.  There are current issues with
non-singleton groups that userspace doesn't handle (see "fumble
through" comment above), but I'm not willing to say "this is
userspace's problem anyway, therefore it's ok it make it worse".

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For in-kernel users, we're still theoretically trying to isolate
devices such that they have restricted access to only the resources
they need.  Disabling things like ACS in the topology reduces that
isolation.  AFAICT, most users don't really care about that degree of
isolation, so they run with iommu=pt for native driver performance
while still having the IOMMU available for isolation use cases running
in parallel.  We don't currently have support for on-demand enabling
isolation.    

Ok.
  

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The host side nVidia GPGPU
drivers also won't want to isolate the (host owned) NVLink devices
from each other, since they'll want to use the fast interconnects    

This falls into the same mixed use case scenario above where we don't
really have a good solution today.  Things like ACS are dynamically
configurable, but we don't expose any interfaces to let drivers or
users change it (aside from setpci, which we don't account for
dynamically).  We assume a simplistic model where if you want IOMMU,
then you must also want the maximum configurable isolation.
Dynamically changing routing is not necessarily the most foolproof
thing either with potentially in-flight transactions and existing DMA
mappings, which is why I've suggested a couple times that perhaps we
could do a software hot-unplug of a sub-hierarchy, muck with isolation
at the remaining node, then re-discover the removed devices.    

Ok, so I feel like we need to go fully one way or the other.  Either:

1) Groups represent current isolation status, in which case we
   deprecate vfio containers in favour of fusing groups beforehand,
   and we need some new concept ("isolation atoms"?) to represent what
   isolation is possible  

Nope, containers are owned by users and serve a purpose beyond what
you're assuming here.  Also, there's 7.5 years of userspace tooling
broken by this.  I do remember we had discussions about merging groups,
but what we have now is what we agreed on.  

Well, quite - and I thought the reasoning that led to that design was
that groups would be a representation of isolation granularity, not
actual isolation.  That allows them to be static, which as you say,
tools are likely to assume.

What is the value of this idea of potential isolation?  Any device
might be considered potentially isolated, simply disable all the
devices around it.  So now we have a "potentially isolated" group at
each device.  What have we accomplished?  How do we build that into a
user interface?  Who decides where the actual isolation lines are
drawn?  Not that it didn't happen, but I don't recall this design idea
and I don't see how it's practical.

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or

2) Groups represent potential isolation, with a higher level construct
   representing current isolation.  This could involve making vfio
   containers essentially a wrapper around some more generic concept
   ("isolation clusters"?) of a group of groups.  

Have fun.  Containers and groups are exposed to userspace, so you're
essentially suggesting to throw away all that support, for... I don't
really know what.  

No, I'm not proposing removing containers, just changing them from
being purely VFIO specific to being the VFIO front end to a generic
concept.  Roughly speaking VFIO containers would be to "isolation
clusters" (or whatever) as VFIO groups are to IOMMU groups now.

Renaming everything doesn't help.  AFAIK, there's nothing in vfio that
prevents groups from being dynamic (changing them while in use might be
unkind though), but they must be isolated from other groups.  We can't
move the line where we have isolation to anything above the group.
Groups are currently not dynamic because "it's hard" (tm), but
obviously it can be done with device removal and rediscovery.
 

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Groups are involved in IOMMU context, so dynamically
changing what a group defines is hard.  

Absolutely!  That's why I don't want to, and why my conception of
groups was defined so that they didn't have to.

As above, groups are the unit of userspace ownership and therefore must
be isolated.

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Thus my suggestions that mixed
or dynamic workloads could make use of soft remove and rediscovery for
sub-hierarchies, unbinding and rebinding drivers such that we have the
proper expectations of DMA context.   Thanks,  

Well, if we have to change groups, then that sounds like the sanest
available way to do it.  But I'm not yet convinced that altering
groups makes sense rather than using a group-of-groups concept on top
of it - which would map to containers in the case of VFIO.

Groups must be isolated in the vfio API.  What happens underneath a
group might be fungible, but the exposed group must be isolated.
Thanks,

Alex