Re: [Virtio-fs] [PATCH v4 2/8] vhost-user.rst: Improve [GS]ET_VRING_BASE doc

[Hanna Czenczek <hreitz@redhat.com>]

On 06.10.23 15:55, Hanna Czenczek wrote:
> On 06.10.23 10:49, Michael S. Tsirkin wrote:
>> On Fri, Oct 06, 2023 at 09:53:53AM +0200, Hanna Czenczek wrote:
>>> On 05.10.23 19:38, Stefan Hajnoczi wrote:
>>>> On Wed, Oct 04, 2023 at 02:58:58PM +0200, Hanna Czenczek wrote:

[...]

>>>>    ``VHOST_USER_GET_VRING_BASE``
>>>>      :id: 11
>>>>      :equivalent ioctl: ``VHOST_USER_GET_VRING_BASE``
>>>>      :request payload: vring state description
>>>> -  :reply payload: vring state description
>>>> +  :reply payload: vring descriptor index/indices
>>>> +
>>>> +  Stops the vring and returns the current descriptor index or 
>>>> indices:
>>>> +
>>>> +    * For a split virtqueue, returns only the 16-bit next descriptor
>>>> +      index in the *Available Ring*.  The index in the *Used Ring* is
>>>> +      controlled by the guest driver and can be read from the vring
>>>> I find "is controlled by the guest driver" confusing. The device 
>>>> writes
>>>> the Used Ring index. The driver only reads it. The device is the 
>>>> active
>>>> party here.
>>> Er, good point.  That breaks the whole reasoning.  Then I don’t 
>>> understand
>>> why we do get/set the available ring index and not the used ring 
>>> index.  Do
>>> you know why?
>> It's simple. used ring index in memory is controlled by the device and
>> reflects device state.
>
> Exactly, it’s device state, that’s why I thought the front-end needs 
> to ensure its read and restored around the reset we currently have in 
> vhost_dev_stop()/start().
>
>> device can just read it back to restore.
>
> I find it strange that the device is supposed to read its own state 
> from memory.
>
>> available ring index in memory is controlled by driver and does
>> not reflect device state.
>
> Why can’t the device read the available index from memory?  That value 
> is put into memory by the driver precisely so the device can read it 
> from there.

Ah, wait, is the idea that the device may have an internal available 
index counter that reflects what descriptor it has already fetched? I.e. 
this index will lag behind the one in memory, and the difference are new 
descriptors that the device still needs to read? If that internal 
counter is the index that’s get/set here, then yes, that makes a lot of 
sense.

Hanna