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

[Jason Wang <jasowang@redhat.com>]

On Thu, Aug 17, 2023 at 4:43âPM Zhu, Lingshan <lingshan.zhu@intel.com> wrote:
>
>
>
> On 8/17/2023 4:31 PM, Uminski, Piotr wrote:
> >
> >> -----Original Message-----
> >> From: virtio-comment@lists.oasis-open.org <virtio-comment@lists.oasis-
> >> open.org> On Behalf Of Zhu, Lingshan
> >> Sent: Tuesday, August 15, 2023 1:30 PM
> >> To: Jason Wang <jasowang@redhat.com>
> >> Cc: mst@redhat.com; eperezma@redhat.com; cohuck@redhat.com; virtio-
> >> comment@lists.oasis-open.org; virtio-dev@lists.oasis-open.org
> >> Subject: [virtio-comment] Re: [RFC PATCH 4/5] virtqueue: constraints for
> >> virtqueue state
> >>
> >>
> >>
> >> On 8/15/2023 8:34 AM, Jason Wang wrote:
> >>> On Mon, Aug 14, 2023 at 7:29âPM 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
> > It should be possible to set Virtqueue State in two states:
> >   - Before DRIVER_OK - as a part of the queue creation at target system
> > -  After SUSPEND - when the device was activated and then suspended
> > Setting state on active device can produce random results and should be forbidden.
> > We may allow to get Virtqueue State on active device, without SUSPEND. However, we should not mandate the device
> > to report correct value - on some implementations it may be impossible without suspend.

Exactly.

> I agree

Great.

Thanks

> >>>> +
> >>>> +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
> >>>> +
> >>>> +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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