On Thu, 15 Nov 2018 10:05:59 +0000
Daniel P. Berrangé <berrange@xxxxxxxxxx> wrote:
> On Wed, Nov 14, 2018 at 09:38:31PM -0200, Eduardo Habkost wrote:
> > Many of the current virtio-*-pci device types actually represent
> > 3 different types of devices:
> > * virtio 1.0 non-transitional devices
> > * virtio 1.0 transitional devices
> > * virtio 0.9 ("legacy device" in virtio 1.0 terminology)
> >
> > That would be just an annoyance if it didn't break our device/bus
> > compatibility QMP interfaces. With this multi-purpose device
> > type, there's no way to tell management software that
> > transitional devices and legacy devices require a Conventional
> > PCI bus.
> >
> > The multi-purpose device types would also prevent us from telling
> > management software what's the PCI vendor/device ID for them,
> > because their PCI IDs change at runtime depending on the bus
> > where they were plugged.
> >
> > This patch adds separate device types for each of those virtio
> > device flavors:
> >
> > - virtio-*-pci: the existing multi-purpose device types
> > - Configurable using `disable-legacy` and `disable-modern`
> > properties
> > - Legacy driver support is automatically enabled/disabled
> > depending on the bus where it is plugged
> > - Supports Conventional PCI and PCI Express buses
> > (but Conventional PCI is incompatible with
> > disable-legacy=off)
> > - Changes PCI vendor/device IDs at runtime
> > - virtio-*-pci-transitional: virtio-1.0 device supporting legacy drivers
It's a virtio-1 (not 1.0) device. Otherwise, I like this terminology
better.
> > - Supports Conventional PCI buses only, because
> > it has a PIO BAR
>
> Am I right in thinking that this is basically identical
> to virtio-*-pci, aside from only being valid for PCI
> buses ?
>
> IOW, libvirt can expose this device even if QEMU does
> not support it, by simply using the existing device
> type and only ever placing it in a PCI bus ?
>
> If libvirt did this compatibility approach, can you
> confirm this would be live migration state compatible.
>
> ie can live migrate virtio-*-pci -> virtio-*-pci-transitional,
> provided only PCI bus was used.
It also needs to make sure that neither disable-legacy nor
disable-modern is set. Then this would have a compatible state AFAICS.
>
> > - virtio-*-pci-non-transitional: modern-only
> > - Supports both Conventional PCI and PCI Express buses
>
> IIUC, libvirt can again provide compatibility with old
> QEMU by simply using the existing device type and setting
> disable-legacy ? Can you confirm this would be live
> migration compatible
>
> virtio-*-pci + disable-legacy -> virtio-*pci-non-transitional
I think yes.
[Out of curiosity, libvirt does not do anything with virtio-ccw's max
revision attribute, does it? QEMU uses this on a machine-type level for
compat handling, but I don't think it is useful beyond that.
Fortunately, virtio-ccw does not have complications like the
PCI/PCI-Express bus dependency.]
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