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

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Date:   Wed, 24 Jan 2018 11:18:40 +0800
From:   Wei Wang <wei.w.wang@...el.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
CC:     yang.zhang.wz@...il.com, virtio-dev@...ts.oasis-open.org,
        riel@...hat.com, quan.xu0@...il.com, kvm@...r.kernel.org,
        nilal@...hat.com, liliang.opensource@...il.com,
        linux-kernel@...r.kernel.org, mhocko@...nel.org,
        linux-mm@...ck.org, pbonzini@...hat.com, akpm@...ux-foundation.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [virtio-dev] Re: [PATCH v22 2/3] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_VQ

On 01/22/2018 07:25 PM, Wei Wang wrote:
> 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.


Thanks for the comments, and I adopted most of them in the new posted 
v23 patches. The above discussion is the one that I haven't included. If 
you could still see issues in the above analysis, please let me know. 
Thanks.

Best,
Wei