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

[Maxime Coquelin <maxime.coquelin@redhat.com>]

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.

> 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."

> 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
> 
> 
> 
>