Re: [virtio-comment] RE: [PATCH v5 2/7] Introduce admin command set

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jun 23, 2022 at 11:34:42AM +0800, Jason Wang wrote:
> On Thu, Jun 23, 2022 at 10:57 AM Parav Pandit <parav@nvidia.com> wrote:
> >
> >
> > > From: Jason Wang <jasowang@redhat.com>
> > > Sent: Wednesday, June 22, 2022 10:42 PM
> >
> > [..]
> > > > Are you suggesting only feature bits through register-based transport or all
> > > commands via register-based transport?
> > >
> > > For feature bits, if we don't have admin virtqueue as transport,
> > Right.
> > Management tasks (features) done via AQ are negotiated via the AQ (regardless of device type being management dev or managed dev).
> > It is not replacing usual feature bits negotiation that has strong relationships with device initialization sequence.
> 
> Just to clarify,
> 
> E.g if the admin virtqueue is implemented in PF. I suggest using the
> existing PCI transport based feature negotiation since:
> 
> 1) we have selector, so we don't need any new registered and we don't
> need worry about the scalability


I think this is a point that needs addressing.

To be more specific. The concern raised was that feature bits are memory
mapped. Accessing them on PCI requires an MMIO write. According to the
PCI spec it is not legal for a device to defer accepting a write until
another write is accepted (since that leads to deadlocks).
Thus any info programmed using MMIO writes has to reside in
on-card memory and can not be offloaded to system RAM.

After thinking about these issues I have an idea:

At the moment VQs are already never programmed before FEATURES_OK,
features are never programmed after FEATURES_OK.
This implies that device can actually store features in
VQ state registers temporarily until FEATURES_OK.
This is more than 24 bytes of memory per VQ, with at least
one data VQ and one admin VQ, this will be sufficient for a long time.

Thus, all that we need to do is prohibit programming VQs
before FEATURES_OK more strongly.
I can work on that if the idea is acceptable to others.




-- 
MST