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

[Matan Azrad <matan@mellanox.com>]

From: Maxime Coquelin:
> On 6/23/20 11:02 AM, Matan Azrad wrote:
> >
> >
> > From: Maxime Coquelin:
> >> On 6/22/20 5:51 PM, Matan Azrad wrote:
> >>>
> >>>
> >>> From: Maxime Coquelin:
> >>>> On 6/22/20 3:43 PM, Matan Azrad wrote:
> >>>>>
> >>>>>
> >>>>> From: Maxime Coquelin:
> >>>>>> Sent: Monday, June 22, 2020 3:33 PM
> >>>>>> To: Matan Azrad <matan@mellanox.com>; Xiao Wang
> >>>>>> <xiao.w.wang@intel.com>
> >>>>>> 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:
> >>>>>>>
> >>>>>>> Hi Maxime
> >>>>>>>
> >>>>>>> From: Maxime Coquelin <maxime.coquelin@redhat.com>
> >>>>>>>> Sent: Monday, June 22, 2020 11:56 AM
> >>>>>>>> To: Matan Azrad <matan@mellanox.com>; Xiao Wang
> >>>>>>>> <xiao.w.wang@intel.com>
> >>>>>>>> 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:
> >>>>>>>>>> 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.
> >>>>>>>>> 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
> >>>>>>>> **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.
> >>>>>>>
> >>>>>>> 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.
> >>>>>>> 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
> >>>>>> already ready.
> >>>>>>> So when driver/app see state enable->enable, it should take into
> >>>>>>> account
> >>>>>> that the queue configuration was probably changed.
> >>>>>>>
> >>>>>>> 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.
> >>>>>>
> >>>>>>> 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:
> >>>>>>> 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.
> >>>>> 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?
> >
> > Even after initialization when queue is ready.
> >
> >>>
> >>>>> 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?
> >>>>> 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.
> >
> > I understand your point here but do you really want to be ready for any
> configuration update in run time?
> > What does it mean? How datatpath should handle configuration from
> control thread in run time while traffic is on?
> > 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.

> > 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.

> 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.

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

> 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.
But in case the queue is ready, your proposal send extra not ready notification for kikfd,callfd,set_vring_base configurations.

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

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


I will not insist anymore on waiting for enable for notifying although I not fan with it.

So, I suggest to create 1 notification function to be called after message handler and before reply.
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.


Matan



> Maxime