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

[Laine Stump]

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
>>> 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.

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).


>
> 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.