Re: [RFC] virtio_scsi: to poll and kick the virtqueue in timeout handler

[Stefan Hajnoczi]

On Mon, May 24, 2021 at 11:33:33PM -0700, Dongli Zhang wrote:
> On 5/24/21 6:24 AM, Stefan Hajnoczi wrote:
> > On Sun, May 23, 2021 at 09:39:51AM +0200, Hannes Reinecke wrote:
> >> On 5/23/21 8:38 AM, Dongli Zhang wrote:
> >>> This RFC is to trigger the discussion about to poll and kick the
> >>> virtqueue on purpose in virtio-scsi timeout handler.
> >>>
> >>> The virtio-scsi relies on the virtio vring shared between VM and host.
> >>> The VM side produces requests to vring and kicks the virtqueue, while the
> >>> host side produces responses to vring and interrupts the VM side.
> >>>
> >>> By default the virtio-scsi handler depends on the host timeout handler
> >>> by BLK_EH_RESET_TIMER to give host a chance to perform EH.
> >>>
> >>> However, this is not helpful for the case that the responses are available
> >>> on vring but the notification from host to VM is lost.
> >>>
> >> How can this happen?
> >> If responses are lost the communication between VM and host is broken, and
> >> we should rather reset the virtio rings themselves.
> > 
> > I agree. In principle it's fine to poll the virtqueue at any time, but I
> > don't understand the failure scenario here. It's not clear to me why the
> > device-to-driver vq notification could be lost.
> > 
> 
> One example is the CPU hotplug issue before the commit bf0beec0607d ("blk-mq:
> drain I/O when all CPUs in a hctx are offline") was available. The issue is
> equivalent to loss of interrupt. Without the CPU hotplug fix, while NVMe driver
> relies on the timeout handler to complete inflight IO requests, the PV
> virtio-scsi may hang permanently.
> 
> In addition, as the virtio/vhost/QEMU are complex software, we are not able to
> guarantee there is no further lost of interrupt/kick issue in the future. It is
> really painful if we encounter such issue in production environment.

Any number of hardware or software bugs might exist that we don't know
about, yet we don't pre-emptively add workarounds for them because where
do you draw the line?

I checked other SCSI/block drivers and found it's rare to poll in the
timeout function so there does not seem to be a consensus that it's
useful to do this.

That said, it's technically fine to do it, the virtqueue APIs are there
and can be used like this. So if you and others think this is necessary,
then it's a pretty small change and I'm not against merging a patch like
this.

Stefan

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