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Date: Tue, 28 Apr 2009 11:51:54 +0200
From: Christoph Hellwig <hch@lst.de>
Subject: Re: [Qemu-devel] Re: [PATCH] virtio-blk: add SGI_IO passthru support
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: kvm@vger.kernel.org, Rusty Russell <rusty@rustcorp.com.au>, qemu-devel@nongnu.org, Christian Borntraeger <borntraeger@de.ibm.com>, Hannes Reinecke <hare@suse.de>, Christoph Hellwig <hch@lst.de>

On Mon, Apr 27, 2009 at 09:36:51AM -0500, Anthony Liguori wrote:
> So practically speaking, what can you do with this?

It's interesting if your underlying device is a real scsi disk and you
want virtio for efficiency but also allow to issue scsi commands on
the underlying device, e.g. for finding out more information about
the underlying setup using the sg_* tools or for certain manual
multipathing setups, or for upgrading firmware or..

> Should we be 
> handling some SCSI cmds internally to QEMU (like eject operations) and 
> supporting media=cdrom in -drive for if=virtio?

Not quite yet.  Eventually I want to support a virtio-scsi kind of
transport which would use the same virtio-blk protocol but only send
scsi commands.  We'd need a different driver on the Linux side for
it that registers to the scsi layer.  On the QEMU side it could either
do pass-through to a real scsi device using SG_IO or use the existing
command scsi emulator in scsi-disk.c.

> On a related topic, should we switch /dev/vdX to be /dev/sdX?

We can't and don't want for the current virtio-blk driver.  Once we
implement a virtio-scsi driver in the guest disk will show up as
/dev/sdX.