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

[Paolo Bonzini]

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?

Paolo