Re: [Qemu-devel] [PATCH 1/2 v1] blkdrv: Add queue limits parameters for sg block drive

[Stefan Hajnoczi]

On Thu, Aug 23, 2012 at 11:03 AM, Paolo Bonzini <address@hidden> wrote:
> Il 23/08/2012 11:31, Cong Meng ha scritto:
>>> For disks, this should be fixed simply by using scsi-block instead of
>>> scsi-generic.
>>>
>>> CD-ROMs are indeed more complicated because burning CDs cannot be done
>>> with syscalls. :/
>>
>> So, as the problem exist to CD-ROM, I will continue to get these patches
>> move on.
>
> I'm still trying to understand the extent of the problem.
>
> The problem occurs for _USB_ CD-ROMs according to Ben.  Passthrough of
> USB storage devices should be done via USB passthrough, not virtio-scsi.
>  If we do USB passthrough via the SCSI layer we miss on all the quirks
> that the OS may do based on the USB product/vendor pairs.  There's no
> end to these, and some of the quirks may cause the device to lock up or
> corruption.
>
> I'd rather see a reproducer using SAS/ATA/ATAPI disks before punting.

This issue affects passthrough: either an entire sg device or at least
a SG_IO ioctl (e.g. a non-READ/WRITE SCSI command).

To reproduce it, check host queue limits and guest virtio-scsi queue
limits.  Then pick a command that can exceed the limits and try it
from inside the guest :).

Stefan