[virtio-dev] Re: [RFC PATCH 4/5] virtqueue: constraints for virtqueue state

["Zhu, Lingshan" <lingshan.zhu@intel.com>]

On 8/17/2023 11:19 PM, Eugenio Perez Martin wrote:
> On Tue, Aug 15, 2023 at 1:30 PM Zhu, Lingshan <lingshan.zhu@intel.com> wrote:
>>
>>
>> On 8/15/2023 8:34 AM, Jason Wang wrote:
>>> On Mon, Aug 14, 2023 at 7:29 PM Zhu Lingshan <lingshan.zhu@intel.com> wrote:
>>>> This commit specifies the constraints of the virtqueue state,
>>>> and the actions should be taken by the device when SUSPEND
>>>> and DRIVER_OK is set
>>>>
>>>> Signed-off-by: Jason Wang <jasowang@redhat.com>
>>>> Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
>>>> ---
>>>>    content.tex | 31 +++++++++++++++++++++++++++++++
>>>>    1 file changed, 31 insertions(+)
>>>>
>>>> diff --git a/content.tex b/content.tex
>>>> index 43bd5de..f6ac581 100644
>>>> --- a/content.tex
>>>> +++ b/content.tex
>>>> @@ -587,6 +587,37 @@ \subsection{\field{Used State} Field}
>>>>
>>>>    See also \ref{sec:Packed Virtqueues / Driver and Device Ring Wrap Counters}.
>>>>
>>>> +\drivernormative{\subsection}{Virtqueue State}{Basic Facilities of a Virtio Device / Virtqueue State}
>>>> +
>>>> +If VIRTIO_F_QUEUE_STATE has been negotiated, the driver MUST set SUSPEND in \field{device status}
>>>> +first before getting or setting Virtqueue State of any virtqueues.
>>> I don't get why this is a must. It could be useful for debugging.
>> To avoid race conditions with the device and make the device
>> implementation easier
>>>> +
>>>> +If VIRTIO_F_QUEUE_STATE has been negotiaged but VIRTIO_RING_F_PACKED not been negotiated,
>>> typo
>> yes
>>>> +the driver MUST NOT access \field{Used State} of any virtqueues, it should use the
>>>> +used index in the used ring.
>>>> +
>>>> +\devicenormative{\subsection}{Virtqueue State}{Basic Facilities of a Virtio Device / Virtqueue State}
>>>> +
>>>> +If VIRTIO_F_QUEUE_STATE has been negotiated but SUSPEND is not set in \field{device status},
>>>> +the device MUST ignore any accesses against Virtqueue State of any virtqueues.
>>> Btw, do we need to clarify the behavior of ring reset after suspending?
>> I think once suspended, the device should ignore resetting a queue
> Actually shadow virtqueue could benefit from the ability to change vq
> properties (addresses) while the device is suspended, and then just
> resume it. I've been told that ring reset is overkill for that.
If ring reset is overkill, is SUSPEND even more overkill?
>
> But probably it is better to address it on top, with another feature flag.
I think if we want to changing the vq properties, there must be a 
mechanism to
stop the queue then resume the queue.

How about allow setting queue_enable = 0 to stop it and =1 to resume and
force it reinitialize?

Thanks
Zhu Lingshan
>
>>>> +
>>>> +When VIRTIO_F_QUEUE_STATE has been negotiated but VIRTIO_RING_F_PACKED is not,
>>>> +the device MUST ignore any accesses against \field{Used State}.
>>>> +
>>>> +If VIRTIO_F_QUEUE_STATE has been negotiaged, the device MUST reset
>>>> +the Virtqueue State of every virtqueue upon a reset.
>>> Need to define the meaning of "reset" this is important for packed virtqueue.
>> I will remove this as Stefan suggested.
>>>> +
>>>> +If VIRTIO_F_QUEUE_STATE and VIRTIO_RING_F_PACKED have been negotiaged, when SUSPEND is set,
>>>> +the device MUST record the Virtqueue State of every enabled virtqueue
>>>> +in \field{Available State} and \field{Used State} respectively,
>>>> +and correspondingly restore the Virtqueue State of every enabled virtqueue
>>>> +from \field{Avaiable State} and \field{Used State} when DRIVER_OK is set.
>>> We can just let the device report those states in any case then we
>>> don't need to care about those details, or did you see any blockers?
>> Agree, I will add the definition of used_state of splitted vq in the
>> next version
>>
>> Thanks
>>> Thanks
>>>
>>>> +
>>>> +If VIRTIO_F_QUEUE_STATE has been negotiated but VIRTIO_RING_F_PACKED has been not, when SUSPEND is set,
>>>> +the device MUST record the available state of every enabled virtqueue in \field{Available State},
>>>> +and restore the available state of every enabled virtqueue from \field{Avaiable State}
>>>> +when DRIVER_OK is set.
>>>> +
>>>>    \input{admin.tex}
>>>>
>>>>    \chapter{General Initialization And Device Operation}\label{sec:General Initialization And Device Operation}
>>>> --
>>>> 2.35.3
>>>>


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