Re: [Qemu-devel] [PATCH 2/2] virtio-scsi/virtio-blk: Disable poll handlers when stopping vq handler

[Fam Zheng <famz@redhat.com>]

On Wed, 09/12 14:42, Paolo Bonzini wrote:
> On 12/09/2018 13:50, Fam Zheng wrote:
> >> 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
> 
> But with iothread, virtio_scsi_dataplane_stop runs in another thread
> than the iothread; in that case you still have a race where the iothread
> can process the vq before aio_disable_external and print the error.
> 
> IIUC the guest has cleared the IOMMU page tables _before_ clearing the
> DRIVER_OK bit in the status field.  Could this be a guest bug?
> 

I'm not sure if it is a bug or not. I think what happens is the device is left
enabled by Seabios, and then reset by kernel.

Fam