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

[Matan Azrad <matan@mellanox.com>]

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.

 
> > 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.
As I mentioned, QEMU sends enable message always after configuration message.


> > 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
> >
> >
> >
> >