[libvirt] [PATCHv5 00/13] qemu: allow disabling certain virtio revisions

[Cornelia Huck]

[I've browsed through the thread a bit, but as I'm not a libvirt
developer I may be missing some basic things]

On Wed, 7 Sep 2016 16:34:17 -0400
Laine Stump <laine at laine.org> wrote:

> On 09/07/2016 03:38 PM, Sascha Silbe wrote:
> > Dear Laine,
> >
> > Laine Stump <laine at laine.org> writes:
> >
> >> On 09/07/2016 02:35 PM, Sascha Silbe wrote:
> >>> "Daniel P. Berrange" <berrange at redhat.com> writes:
> >>> [...]
> >>>>        <sound model="virtio"/>     == QEMU virtio
> >>>>        <sound model="virtio1.0"/>  == QEMU virtio + disable-legacy

I'm not a fan of the virtio1.0 name, but that has already been
commented upon elsewhere.

> >>> What would this do for devices using the virtio-ccw transport?
> >>   From libvirt's point of view, the option "disable-legacy=on" would be
> >> added to the device's commandline argument.
> > Which would break s390x guests. virtio-ccw doesn't have any concept of
> > "legacy" or "modern" devices (that's purely a virtio-pci concept), so
> > virtio-*-ccw devices don't recognise that switch:
> 
> Okay, so you already know what would happen in qemu. Looking at Jan's 
> code in this patch series, (which I didn't do before, but should have) 
> when someone tries to set the option for disable-legacy=on when the 
> device address is anything except PCI , it logs an error and fails.

Can't you make this a virtio-pci only switch? Or is that problem
occurring when expanding a generic virtio device?

> 
> No code for Dan's suggestion has been written yet, but if there's no 
> concept of a legacy mode for virtio-*-ccw, then we would do the same 
> thing. And also I would guess that libosinfo would never suggest that 
> anyone try to add a "virtio1.0" model device to an s390 virtual machine).

The thing is that, unlike virtio-pci, we don't have any reason to
actually disable support for legacy virtio devices, and therefore don't
have a switch for it.

> 
> 
> >
> > silbe at oc4731375738:~$ ~/build/qemu-devel/x86_64-softmmu/qemu-system-x86_64 -device virtio-blk,help 2>&1 |grep legacy
> > virtio-blk-pci.disable-legacy=OnOffAuto (on/off/auto)
> > silbe at oc4731375738:~$ ~/build/qemu-devel/s390x-softmmu/qemu-system-s390x -device virtio-blk,help 2>&1 |grep legacy
> >
> > That nicely illustrates the issue I have with a) mixing virtio-pci
> > legacy/modern into the model name and b) conflating it with virtio
> > 0.9/1.0 (or transitional/non-transitional for that matter).
> >
> > FWIW, the thing closest to virtio-pci legacy/modern is virtio-ccw
> > max_revision. But I doubt there's any reason to set this beyond
> > debugging and testing.
> 
> Definitely - once we've added an option to libvirt, we have to keep it 
> there forever - our backward compatibility guarantee requires it. So we 
> don't want to add anything unless there's a clear use for it.

I don't see any reason why you'd want to make max_revision configurable
from libvirt. QEMU using it for compat machines is the only reason for
it.