Re: [virtio-dev] Re: [PATCH v3] virtio-mmio: Specify wait needed in driver during reset

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

On Tue, Aug 31, 2021 at 09:26:03PM +0530, Srivatsa Vaddagiri wrote:
> * Michael S. Tsirkin <mst@redhat.com> [2021-08-31 10:45:53]:
> 
> > On Tue, Aug 31, 2021 at 07:27:53PM +0530, Srivatsa Vaddagiri wrote:
> > > Reset of a virtio-mmio device is initiated by writing 0 to its Status register.
> > > In case of some devices, the reset operation itself may not be completed
> > > by the time write instruction completes and hence such devices would require
> > > drivers to wait on reset operation to complete before they proceed with
> > > remaining steps of initialization.
> > > 
> > > Update the specification to indicate which devices would need driver to block on
> > > reset completion.
> > > 
> > > Suggested-by: Jason Wang <jasowang@redhat.com>
> > > Signed-off-by: Srivatsa Vaddagiri <quic_svaddagi@quicinc.com>
> > 
> > 
> > I am still of two minds on whether we
> > want such a drastic change as a version update for such a
> > minor thing. Yes, we did it for PCI but then PCI did
> > not break backwards compat like mmio did.
> > 
> > Let's see what needs to happen to make existing drivers work
> > 1- reset starts the reset process
> > 2- following writes into status are buffered by the device
> >   until reset completes
> > 3- read from features completes after reset is complete
> 
> Couple of scenarios which we discussed in this regard earlier:
> 
> 1) What if device reset encounters a failure? What should it return for the
> 'features' read in that case?

I'd say the main thing is to fail to set FEATURES_OK.

> 2) For untrusted devices, like in our case [A], it would require hypervisor to
> stall vcpu until the untrusted backend responds to the features read request,

Wait a second, this is fundamental to reads anyway. They can't bypass
writes. E.g. this is the case when FEATURES_OK is written then read
back.

> which could take a long time.

This last is a reasonable argument. so it's not about hardware it's for
software where reset takes a long time and we do not
want to stall the VCPU. That's a reasonable requirement but
pls include it in the text though. And I wonder how you are handling
other cases where reads are ordered with writes.
Is there a reason to assume reset is special?


>  In worst case, the VM may get reset due to
> watchdog firing. Requiring drivers to poll will avoid that situation and allow
> drivers to fail probe gracefully.
>
> Ref A: https://lists.oasis-open.org/archives/virtio-dev/202108/msg00090.html
> 
> - vatsa
> 
> -- 
> Qualcomm Innovation Center, Inc. is submitting the attached "feedback" as a
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> 
> 
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