Re: [dpdk-dev] [PATCH v1 3/4] vhost: improve device ready definition

From: Maxime Coquelin <hidden>
Date: 2020-06-24 09:13:11


On 6/24/20 10:38 AM, Matan Azrad wrote:

quoted

-----Original Message-----
From: Maxime Coquelin <redacted>
Sent: Wednesday, June 24, 2020 10:22 AM
To: Matan Azrad <redacted>; Xiao Wang
[off-list ref]
Cc: dev@dpdk.org
Subject: Re: [PATCH v1 3/4] vhost: improve device ready definition

Good morning Matan,

On 6/24/20 7:54 AM, Matan Azrad wrote:

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Ho Maxime

Good morning

From: Maxime Coquelin:

quoted

On 6/23/20 4:52 PM, Matan Azrad wrote:

quoted

-----Original Message-----
From: Maxime Coquelin <redacted>
Sent: Tuesday, June 23, 2020 4:56 PM
To: Matan Azrad <redacted>; Xiao Wang
[off-list ref]
Cc: dev@dpdk.org
Subject: Re: [PATCH v1 3/4] vhost: improve device ready definition

Hi Matan,

On 6/23/20 1:53 PM, Matan Azrad wrote:

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From: Maxime Coquelin:

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On 6/23/20 11:02 AM, Matan Azrad wrote:

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From: Maxime Coquelin:

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On 6/22/20 5:51 PM, Matan Azrad wrote:

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From: Maxime Coquelin:

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On 6/22/20 3:43 PM, Matan Azrad wrote:

quoted


From: Maxime Coquelin:

quoted

Sent: Monday, June 22, 2020 3:33 PM
To: Matan Azrad <redacted>; Xiao Wang
[off-list ref]
Cc: dev@dpdk.org
Subject: Re: [PATCH v1 3/4] vhost: improve device ready
definition



On 6/22/20 12:06 PM, Matan Azrad wrote:

quoted

Hi Maxime

From: Maxime Coquelin <redacted>

quoted

Sent: Monday, June 22, 2020 11:56 AM
To: Matan Azrad <redacted>; Xiao Wang
[off-list ref]
Cc: dev@dpdk.org
Subject: Re: [PATCH v1 3/4] vhost: improve device ready
definition



On 6/22/20 10:41 AM, Matan Azrad wrote:

quoted

The issue is if you only check ready state only before
and after the message affecting the ring is handled, it
can be ready at both stages, while the rings have
changed and state change callback should

have been called.

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But in this version I checked twice, before message
handler and after

message handler, so it should catch any update.

No, this is not enough, we have to check also during some
handlers, so that the ready state is invalidated because
sometimes it will be ready before and after the message
handler but

with different values.

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That's what I did in my example patch:
@@ -1847,15 +1892,16 @@

vhost_user_set_vring_kick(struct

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virtio_net

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**pdev, struct VhostUserMsg *msg,

...

        if (vq->kickfd >= 0)
                close(vq->kickfd);
+
+       vq->kickfd = VIRTIO_UNINITIALIZED_EVENTFD;
+
+       vhost_user_update_vring_state(dev, file.index);
+
        vq->kickfd = file.fd;


Without that, the ready check will return ready before
and after the kickfd changed and the driver won't be

notified.

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The driver will be notified in the next
VHOST_USER_SET_VRING_ENABLE

message according to v1.

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One of our assumption we agreed on in the design mail is
that it doesn't

make sense that QEMU will change queue configuration

without

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enabling the queue again.

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Because of that we decided to force calling state callback
again when

QEMU send VHOST_USER_SET_VRING_ENABLE(1) message

even

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if

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the

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queue is

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already ready.

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So when driver/app see state enable->enable, it should
take into account

that the queue configuration was probably changed.

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I think that this assumption is correct according to the
QEMU

code.

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Yes, this was our initial assumption.
But now looking into the details of the implementation, I
find it is even cleaner & clearer not to do this assumption.

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That's why I prefer to collect all the ready checks
callbacks (queue state and

device new\conf) to one function that will be called after
the message
handler:

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Pseudo:
 vhost_user_update_ready_statuses() {
	switch (msg):
		case enable:
			if(enable is 1)
				force queue state =1.
		case callfd
		case kickfd
				.....
		Check queue and device ready + call callbacks

if

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needed..

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		Default
			Return;
}

I find it more natural to "invalidate" ready state where it
is handled (after vring_invalidate(), before setting new FD
for call & kick, ...)

I think that if you go with this direction, if the first
queue pair is invalidated,

you need to notify app\driver also about device ready change.

quoted

Also it will cause 2 notifications to the driver instead of
one in case of FD

change.

You'll always end-up with two notifications, either Qemu has
sent the disable and so you'll have one notification for the
disable and one for the enable, or it didn't sent the disable
and it will happen at old value invalidation time and after
new value is taken into

account.

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I don't see it in current QEMU behavior.
When working MQ I see that some virtqs get configuration

message

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while

they are in enabled state.

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Then, enable message is sent again later.

I guess you mean the first queue pair? And it would not be in
ready state as it would be the initial configuration of the queue?

Even after initialization when queue is ready.

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Why not to take this correct assumption and update ready
state only in one

point in the code instead of doing it in all the
configuration handlers

around?

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IMO, It is correct, less intrusive, simpler, clearer and cleaner.

I just looked closer at the Vhost-user spec, and I'm no more
so sure this is a correct assumption:

"While processing the rings (whether they are enabled or
not), client must support changing some configuration aspects
on the

fly."

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Ok, this doesn't explain how configuration is changed on the fly.

I agree it lacks a bit of clarity.

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As I mentioned, QEMU sends enable message always after
configuration

message.

Yes, but we should not do assumptions on current Qemu version
when possible. Better to be safe and follow the specification,
it will be more

robust.

quoted

There is also the Virtio-user PMD to take into account for

example.

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I understand your point here but do you really want to be ready
for any

configuration update in run time?

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What does it mean? How datatpath should handle configuration
from

control thread in run time while traffic is on?

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For example, changing queue size \ addresses must stop traffic

before...

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Also changing FDs is very sensitive.

It doesn't make sense to me.

Also, according to "on the fly" direction we should not disable
the queue

unless enable message is coming to disable it.

No response, so looks like you agree that it doesn't make sense.

No, my reply was general to all your comments.

With SW backend, I agree we don't need to disable the rings in case
of asynchronous changes to the ring because we protect it with a
lock, so we are sure the ring won't be accessed by another thread
while doing the change.

For vDPA case that's more problematic because we have no such
locking mechanism.

For example memory hotplug, Qemu does not seem to disable the

queues

quoted

so we need to stop the vDPA device one way or another so that it
does not process the rings while the Vhost lib remaps the memory

areas.

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In addition:
Do you really want to toggle vDPA drivers\app for any
configuration

message? It may cause queue recreation for each one (at least for

mlx5).

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I want to have something robust and maintainable.

Me too.

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These messages arriving after a queue have been configured once
are rare events, but this is usually the kind of things that
cause maintenance

burden.

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In case of guest poll mode (testpmd virtio) we all the time get
callfd

twice.

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Right.

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If you look at my example patch, you will understand that with my
proposal, there won't be any more state change notification than
with your proposal when Qemu or any other Vhost-user master

send

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a disable request before sending the request that impact the
queue

state.

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we didn't talk about disable time - this one is very simple.

Yes, In case the queue is disabled your proposal doesn't send
extra

notification as my.

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But in case the queue is ready, your proposal send extra not ready

notification for kikfd,callfd,set_vring_base configurations.

I think this is necessary for synchronization with the Vhost-user
master (in case the master asks for this synchronization, like
set_mem_table for instance when reply-ack is enabled).

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It just adds more robustness if this unlikely event happens, by
invalidating the ring state to not ready before doing the actual
ring

configuration change.

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So that this config change is not missed by the vDPA driver or
the

application.

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One more issue here is that there is some time that device is
ready (already

configured) and the first vittq-pair is not ready (your invalidate
proposal for set_vring_base).

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It doesn’t save the concept that device is ready only in case the
first virtq-

pair is ready.

I understand the spec as "the device is ready as soon as the first
queue pair is ready", but I might be wrong.

Do you suggest to call the dev_close() vDPA callback and the
destroy_device() application callback as soon as one of the ring of
the first queue pair receive a disable request or, with my patch,
when one of the rings receives a request that changes the ring state?

I means, your proposal actually may make first virtq-pair ready
state

disabled when device ready.

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So, yes, it leads to call device close\destroy.

No it doesn't, there is no call to .dev_close()/.destroy_device()
with my patch if first queue pair gets disabled.

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I will not insist anymore on waiting for enable for notifying
although I not

fan with it.

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So, I suggest to create 1 notification function to be called after
message

handler and before reply.

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This function is the only one which notify ready states in the
next

options:

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1. virtq ready state is changed in the queue.
2. virtq ready state stays on after configuration message handler.
3. device state will be enabled when the first queue pair is ready.

IIUC, it will not disable the queues when there is a state change,
is that correct? If so, I think it does not work with memory
hotplug case I mentioned earlier.

It will do enable again which mean - something was modified.

Ok, thanks for the clarification.

I think it is not enough for the examples I gave below. For
set_mem_table, we need to stop the device from processing the vrings
before the set_mem_table handler calls the munmap(), and re-enable it
after the
mmap() (I did that wrong in my example patch, I just did that after
the munmap/mmap happened, which is too late).

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Even for the callfd double change it can be problematic as
Vhost-lib will close the first one while it will still be used by
the driver (Btw, I see my example patch is also buggy in this
regards, it should reset the call_fd value in the virtqueue, then
call
vhost_user_update_vring_state() and finally close the FD).

Yes, this one leads for different handle for each message.

Maybe it leads for new queue modify operation.
So, queue doesn't send the state - just does configuration change on
the

fly.

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What do you think?

I think that configuration on the fly doesn't fly.
We would at least need to stop the device from processing the rings
for memory hotplug case, so why not just send a disable notification?

Yes, driver need notification here.

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And for the double callfd, that does not look right to me not to
request the driver to stop using it before it is closed, isn't it?

Yes, and some drivers (include mlx5) may stop the traffic in this case too.

modify\update operation will solve all:

For example:

In memory hotplug:
Do new mmap
Call modify
Do munmup for old.

In callfd\kickfd change:

Set new FD.
Call modify.
Close old FD.

Modify is clearer, save calls and faster (datapath will back faster).

It should work, but that is not light modifications to do in set_mem_table
handler (the function is quite complex already with postcopy live-migration
support).

With a modify callback, won't the driver part be more complex? Since it
would have to check which state has changed in the ring, and based on that
decide whether it should stop the ring or not.

As you says that in case of memory hotplug and double callfd, the driver may
stop processing the rings anyway, so would it be that much faster than
disabling/enabling the vring?

These events having a very rare occurrence, does it really matter if it is a bit
longer?


Just thinking again about memory hotplug:

Mlx5 driver device need to be reinitialized in this case because the NIC has memory translation which must be updated before the virtqs creation.

So, maybe we need to close and config the vDPA device in this case.

Right, disabling vrings is not enough for memory hotplug.

It would make sense to call dev_close and dev_conf here, that's the most
conservative approach.

@Xiao Wang, can you comment the IFC behavior here.

Matan

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Thanks,
Maxime
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 Thanks,
Maxime
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Thanks,
Maxime
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Matan
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Maxime

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