Re: [libvirt] [PATCH for-4.0 v2] virtio: Provide version-specific variants of virtio PCI devices
On Thu, 15 Nov 2018 10:05:59 +0000
Daniel P. Berrangé <berrange(a)redhat.com> 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.]