On Thu, 2012-08-23 at 11:08 +0100, Stefan Hajnoczi wrote:

On Thu, Aug 23, 2012 at 11:03 AM, Paolo Bonzini [off-list ref] wrote:

quoted

Il 23/08/2012 11:31, Cong Meng ha scritto:

quoted

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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 :).

Just following along on this thread, and wanted to add a few of my
experiences with this scenario from the kernel target perspective..

So up until very recently, TCM would accept an I/O request for an DATA
I/O type CDB with a max_sectors larger than the reported max_sectors for
it's TCM backend (regardless of backend type), and silently generate N
backend 'tasks' to complete the single initiator generated command.
Also FYI for Paolo, for control type CDBs I've never actually seen an
allocation length exceed max_sectors, so in practice AFAIK this only
happens for DATA I/O type CDBs.

This was historically required by the pSCSI backend driver (using a
number of old SCSI passthrough interfaces) in order to support this very
type of case described above, but over the years the logic ended up
creeping into various other non-passthrough backend drivers like IBLOCK
+FILEIO.  So for v3.6-rc1 code, hch ended up removing the 'task' logic
thus allowing backends (and the layers below) to the I/O sectors >
max_sectors handling work, allowing modern pSCSI using struct request to
do the same.  (hch assured me this works now for pSCSI)

Anyways, I think having the guest limit virtio-scsi DATA I/O to
max_sectors based upon the host accessible block limits is reasonable
approach to consider.  Reducing this value even further based upon the
lowest max_sectors available amongst possible migration hosts would be a
good idea here to avoid having to reject any I/O's exceeding a new
host's device block queue limits.

--nab