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Re: [Qemu-devel] [PATCH 2/2] virtio-scsi/virtio-blk: Disable poll handle

From: Fam Zheng
Subject: Re: [Qemu-devel] [PATCH 2/2] virtio-scsi/virtio-blk: Disable poll handlers when stopping vq handler
Date: Wed, 12 Sep 2018 19:50:20 +0800
User-agent: Mutt/1.10.1 (2018-07-13)

On Wed, 09/12 13:11, Paolo Bonzini wrote:
> On 12/09/2018 03:31, Fam Zheng wrote:
> >>>
> >>> ctx is qemu_aio_context here, so there's no interaction with IOThread.
> >> In this case, it should be okay to have the reentrancy, what is the bug
> >> that this patch is fixing?
> > The same symptom as in the previous patch: virtio_scsi_handle_cmd_vq hangs. 
> > The
> > reason it hangs is fixed by the previous patch, but I don't think it should 
> > be
> > invoked as we're in the middle of virtio_scsi_dataplane_stop(). Applying 
> > either
> > one of the two patches avoids the problem, but this one is more superficial.
> > What do you think?
> I think it's okay if it is invoked.  The sequence is first you stop the
> vq, then you drain the BlockBackends, then you switch AioContext.  All
> that matters is the outcome when virtio_scsi_dataplane_stop returns.

Yes, but together with vIOMMU, it also effectively leads to a virtio_error(),
which is not clean. QEMU stderr when this call happens (with patch 1 but not
this patch):

2018-09-12T11:48:10.193023Z qemu-system-x86_64: vtd_iommu_translate: detected 
translation failure (dev=02:00:00, iova=0x0)
2018-09-12T11:48:10.193044Z qemu-system-x86_64: New fault is not recorded due 
to compression of faults
2018-09-12T11:48:10.193061Z qemu-system-x86_64: virtio: zero sized buffers are 
not allowed


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