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

[Srivatsa Vaddagiri <quic_svaddagi@quicinc.com>]

* 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?

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,
which could take a long time.  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

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