Re: [Qemu-devel] virtio-scsi and error handling

[Stefan Hajnoczi <stefanha@gmail.com>]

On Tue, Jun 11, 2013 at 01:41:38PM +0200, Hannes Reinecke wrote:
> I currently playing around with improving SCSI EH, optimizing
> command aborts and the like.
> 
> And, supposing it to be a nice testbed, tried to make things work
> with virtio_scsi.
> 
> However, looking at the code there I've found virtscsi_tmf() just
> uses 'wait_for_completion', with no timeout specified. So in effect
> any abort might stall forever.
> 
> Wouldn't it be more sensible to use 'wait_for_completion_timeout'
> here, to allow the error escalation to continue?
> This would especially be useful when running with multipathing,
> as the underlying device might stall, and aio_cancel() doesn't work
> reliably, if at all.

Hi,
I agree that we need a timeout.  bdrv_aio_cancel() is not guaranteed to
complete in bounded time.

> Also I've found that there is no host reset. Currently the virtio
> semantics seem to require reliable communication, ie for every
> command send there _has_ to be a response.
> 
> Long and painful experience with RAID HBAs has shown that this model
> works okay for the lower-level escalations, but you absolutely need
> a host reset to restore communication.
> In the case of virtio I would think that a virtio-level reset for
> host_reset would be a sensible idea.

One thing to watch out for is that a virtio-scsi reset will likely hang
too because it resets all pending requests.

Paolo Bonzini has done the lion's share of virtio-scsi work over the
past year (or two?).  He might have some more thoughts.

Stefan