Re: [Qemu-devel] [PATCH 2/5] virtio-blk: disable scsi passthrough for 1.0 device

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jul 13, 2015 at 02:30:24PM +0200, Cornelia Huck wrote:
> On Mon, 13 Jul 2015 15:22:52 +0300
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > On Mon, Jul 13, 2015 at 01:51:56PM +0200, Cornelia Huck wrote:
> > > On Mon, 13 Jul 2015 11:56:51 +0200
> > > Kevin Wolf <kwolf@redhat.com> wrote:
> > > 
> > > > Am 13.07.2015 um 11:00 hat Jason Wang geschrieben:
> > > > > 
> > > > > 
> > > > > On 07/13/2015 03:46 PM, Michael S. Tsirkin wrote:
> > > > > > On Mon, Jul 13, 2015 at 01:46:48PM +0800, Jason Wang wrote:
> > > > > >> VIRTIO_BLK_F_SCSI was no longer supported in 1.0. So disable it.
> > > > > >>
> > > > > >> Cc: Stefan Hajnoczi <stefanha@redhat.com>
> > > > > >> Cc: Kevin Wolf <kwolf@redhat.com>
> > > > > >> Cc: qemu-block@nongnu.org
> > > > > >> Signed-off-by: Jason Wang <jasowang@redhat.com>
> > > > > > Interesting, I noticed we have a field scsi - see
> > > > > > 	commit 1ba1f2e319afdcb485963cd3f426fdffd1b725f2
> > > > > > 	Author: Paolo Bonzini <pbonzini@redhat.com>
> > > > > > 	Date:   Fri Dec 23 15:39:03 2011 +0100
> > > > > >
> > > > > > 	    virtio-blk: refuse SG_IO requests with scsi=off
> > > > > >
> > > > > > but it doesn't seem to be propagated to guest features in
> > > > > > any way.
> > > > > >
> > > > > > Maybe we should fix that, making that flag AutoOnOff?
> > > > > 
> > > > > Looks ok but auto may need some compat work since default is true.
> > > > > 
> > > > > > Then, if user explicitly requested scsi=on with a modern
> > > > > > interface then we can error out cleanly.
> > > > > >
> > > > > > Given scsi flag is currently ignored, I think
> > > > > > this can be a patch on top.
> > > > > 
> > > > > Looks like virtio_blk_handle_scsi_req() check this:
> > > > > 
> > > > >     if (!blk->conf.scsi) {
> > > > >         status = VIRTIO_BLK_S_UNSUPP;
> > > > >         goto fail;
> > > > >     }
> > > > 
> > > > So we should be checking the same condition for the feature flag and
> > > > error out in the init function if we have a VERSION_1 device and
> > > > blk->conf.scsi is set.
> > > 
> > > Hm, I wonder how this plays with transports that want to make the
> > > virtio-1 vs. legacy decision post-init? For virtio-ccw, I basically
> > > only want to offer VERSION_1 if the driver negotiated revision >= 1.
> > > I'd need to check for !scsi as well before I can add this feature bit
> > > then? Have the init function set a blocker for VERSION_1 so that the
> > > driver may only negotiate revision 0?
> > 
> > 
> > We already handle this, do we not?
> (...)
> > So guest that doesn't negotiate revision >= 1 never gets to see
> > VIRTIO_F_VERSION_1.
> 
> Not my question :) I was wondering about scsi vs. virtio-1 devices. And
> as I basically only want to make the decision on whether to offer
> VERSION_1 when the guest negotiated a revision, I cannot fence scsi
> during init, no?

No, I don't think there's a lot of value in offering scsi only to
old guests that don't negotiate revision >= 1.

If user asked for virtio 1 support then that by proxy implies scsi
passthrough does not work, and it won't work for legacy
guests too.


> > 
> > Maybe we should go further and additionally all bits >= 32 if
> > VIRTIO_F_VERSION_1 is clear, but that can wait
> > and we have no bits like that in 2.4.
> > 
> Spec says bits >= 32 are only valid if we have VERSION_1, doesn't it?
> Sounds sensible.