linux-kernel - Re: [virtio-dev] Re: [PATCH v22 2/3] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE

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Date:   Mon, 22 Jan 2018 19:25:45 +0800
From:   Wei Wang <wei.w.wang@...el.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
CC:     virtio-dev@...ts.oasis-open.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
        linux-mm@...ck.org, mhocko@...nel.org, akpm@...ux-foundation.org,
        pbonzini@...hat.com, liliang.opensource@...il.com,
        yang.zhang.wz@...il.com, quan.xu0@...il.com, nilal@...hat.com,
        riel@...hat.com
Subject: Re: [virtio-dev] Re: [PATCH v22 2/3] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_VQ

On 01/19/2018 08:39 PM, Michael S. Tsirkin wrote:
> On Fri, Jan 19, 2018 at 11:44:21AM +0800, Wei Wang wrote:
>> On 01/18/2018 12:44 AM, Michael S. Tsirkin wrote:
>>> On Wed, Jan 17, 2018 at 01:10:11PM +0800, Wei Wang wrote:
>>>
>>>> +		vb->start_cmd_id = cmd_id;
>>>> +		queue_work(vb->balloon_wq, &vb->report_free_page_work);
>>> It seems that if a command was already queued (with a different id),
>>> this will result in new command id being sent to host twice, which will
>>> likely confuse the host.
>> I think that case won't happen, because
>> - the host sends a cmd id to the guest via the config, while the guest acks
>> back the received cmd id via the virtqueue;
>> - the guest ack back a cmd id only when a new cmd id is received from the
>> host, that is the above check:
>>
>>      if (cmd_id != vb->start_cmd_id) { --> the driver only queues the
>> reporting work only when a new cmd id is received
>>                          /*
>>                           * Host requests to start the reporting by sending a
>>                           * new cmd id.
>>                           */
>>                          WRITE_ONCE(vb->report_free_page, true);
>>                          vb->start_cmd_id = cmd_id;
>>                          queue_work(vb->balloon_wq,
>> &vb->report_free_page_work);
>>      }
>>
>> So the same cmd id wouldn't queue the reporting work twice.
>>
> Like this:
>
> 		vb->start_cmd_id = cmd_id;
> 		queue_work(vb->balloon_wq, &vb->report_free_page_work);
>
> command id changes
>
> 		vb->start_cmd_id = cmd_id;
>
> work executes
>
> 		queue_work(vb->balloon_wq, &vb->report_free_page_work);
>
> work executes again
>

If we think about the whole working flow, I think this case couldn't happen:

1) device send cmd_id=1 to driver;
2) driver receives cmd_id=1 in the config and acks cmd_id=1 to the 
device via the vq;
3) device revives cmd_id=1;
4) device wants to stop the reporting by sending cmd_id=STOP;
5) driver receives cmd_id=STOP from the config, and acks cmd_id=STOP to 
the device via the vq;
6) device sends cmd_id=2 to driver;
...

cmd_id=2 won't come after cmd_id=1, there will be a STOP cmd in between 
them (STOP won't queue the work).

How about defining the correct device behavior in the spec:
The device Should NOT send a second cmd id to the driver until a STOP 
cmd ack for the previous cmd id has been received from the guest.


Best,
Wei