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Subject: Re: [PATCH v3] virtio-mmio: Specify wait needed in driver during reset
* 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 -- Qualcomm Innovation Center, Inc. is submitting the attached "feedback" as a non-member to the virtio-dev mailing list for consideration and inclusion.
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