Re: [Qemu-devel] [PATCH V2 3/5] virtio-blk: disable scsi passthrough by default

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

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.

> >>> 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.
> 
> Paolo

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?