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Date: Wed, 14 Oct 2015 12:29:17 +0200
From: Cornelia Huck <cornelia.huck@de.ibm.com>
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Subject: Re: [Qemu-devel] [PATCH V2 3/5] virtio-blk: disable scsi
	passthrough by default
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: kwolf@redhat.com, qemu-block@nongnu.org, "Michael S. Tsirkin" <mst@redhat.com>, Jason Wang <jasowang@redhat.com>, qemu-devel@nongnu.org, stefanha@redhat.com

<dragging up an old thread again>

On Wed, 15 Jul 2015 16:45:52 +0200
Paolo Bonzini <pbonzini@redhat.com> wrote:

> On 15/07/2015 16:28, Michael S. Tsirkin wrote:
> > On Wed, Jul 15, 2015 at 04:18:49PM +0200, Paolo Bonzini wrote:
> >>
> >>
> >> On 15/07/2015 16:14, Michael S. Tsirkin wrote:
> >>> On Wed, Jul 15, 2015 at 02:47:24PM +0200, Paolo Bonzini wrote:
> >>>>
> >>>>
> >>>> On 15/07/2015 14:21, Michael S. Tsirkin wrote:
> >>>>>>> Disable scsi passthrough by default since it was incompatible with
> >>>>>>> virtio 1.0. For legacy machine types, keep this on by default.
> >>>>>>>
> >>>>>>> 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>
> >>>>> Seems risky for 2.4.  modern is off by default for now. Can't we limit
> >>>>> the change to when modern is enabled?
> >>>>
> >>>> That would have the effect of disabling a feature when you turn on modern.
> >>>
> >>> What's wrong with that?
> >>
> >> Weren't you complaining about it a few hours ago? :)
> > 
> > No, I complained about guest driver update disabling it.
> 
> Ah, sorry for the confusion.
> 
> >>>>> I suggested changing this from bool to on/off/auto, and
> >>>>> make auto mean !modern.
> >>>>
> >>>> No, please do it like Jason did.  The SCSI feature effectively had to be
> >>>> enabled explicitly already, the requests were marked as unsupported.
> >>>
> >>> I didn't know. How is it enabled?
> >>
> >> It's enabled by default in QEMU, but disabled by default in libvirt.
> >> And it only works if you pass a whole _disk_ (not a partition or logical
> >> volume) to QEMU, which is definitely not the common case.
> >>
> >> It can just be documented in the release notes; the feature is still
> >> available, and libvirt won't be broken because it adds explicitly both
> >> scsi=on and scsi=off.
> > 
> > So for libvirt, we don't really care about the default, right?
> > For command line, would it not be friendlier to make it follow the
> > modern flag automatically?
> 
> I wouldn't mind if we grabbed the occasion to disable it altogether.
> However, that would indeed work as well.

Do we want to change anything for 2.5 about the default?

Currently, we still default scsi to true, and you have to disable it
explicitly if you want to use virtio-1 compliant virtio-blk devices
(which is a bit annoying, as scsi passthrough is not something people
usually want).