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

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-----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:

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From: Maxime Coquelin:

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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:

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Hi Maxime

From: Maxime Coquelin <redacted>

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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:

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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

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.

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
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

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

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.

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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
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."

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.

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

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...
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
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).

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.

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 a disable
request before sending the request that impact the queue state.

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.
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:

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.

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?

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?

Thanks,
Maxime

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Thanks,
Maxime

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Matan

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Maxime