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

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.

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 the

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.

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..
		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.

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.
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?

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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.

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

Thanks,
Maxime

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In addition it saves the style that already used in this function in:
- vhost_user_check_and_alloc_queue_pair
- 	switch (request) {
	case VHOST_USER_SET_FEATURES:
	case VHOST_USER_SET_PROTOCOL_FEATURES:
	case VHOST_USER_SET_OWNER:
	case VHOST_USER_SET_MEM_TABLE:
	case VHOST_USER_SET_LOG_BASE:
	case VHOST_USER_SET_LOG_FD:
	case VHOST_USER_SET_VRING_NUM:
	case VHOST_USER_SET_VRING_ADDR:
	case VHOST_USER_SET_VRING_BASE:
	case VHOST_USER_SET_VRING_KICK:
	case VHOST_USER_SET_VRING_CALL:
	case VHOST_USER_SET_VRING_ERR:
	case VHOST_USER_SET_VRING_ENABLE:
	case VHOST_USER_SEND_RARP:
	case VHOST_USER_NET_SET_MTU:
	case VHOST_USER_SET_SLAVE_REQ_FD:
			vhost_user_lock_all_queue_pairs(dev);

Matan