Re: [libvirt] [PATCH 4/6] qemu: Wire up disk model=virtio-{non-}transitional
On Wed, 2019-01-16 at 11:39 +0000, Daniel P. Berrangé wrote:
On Wed, Jan 16, 2019 at 12:31:04PM +0100, Andrea Bolognani wrote:
> On Tue, 2019-01-15 at 16:56 +0000, Daniel P. Berrangé wrote:
> > A transitional device is 100% identical to the existing device
> > types, so we can simply not add the "-transitional" suffix for
> > old QEMU. The only difference is the way libvirt does PCI bus
> > placement of the transitional device - we'd never use PCIe.
> >
> > A non-transitional device is identical to the existing device
> > types, but with disable-legacy=true set.
>
> Again, the relationship between existing and new devices is not
> quite this straighforward because of the reasons I outlined in
>
> https://www.redhat.com/archives/libvir-list/2019-January/msg00514.html
When told to use virtio-transitional for a device, libvirt would
only plug it into a PCI slot, never a PCI-X slot. Given this
constraint, it is functionally identical / interchangable with
the existing device.
Right, but you didn't spell out the constraint the first time
around, thus making your (broader) statement that a "transitional
device is 100% identical to the existing device" incorrect :)
> But the idea of using disable-{legacy,modern} instead of the
new
> virtio-*-{non,}-transitional devices is one I had already suggested
> in
>
> https://bugzilla.redhat.com/show_bug.cgi?id=1614127
>
> so I'm obviously on board with it :)
>
> > QEMU guarantees this compatibility of the different devices,
> > but only for machine types < pc-i440fx-4.0.0 / pc-q35-4.0.0.
> > So we should none the less make sure we use the modern device
> > names for any QEMU which genuinely supports them.
>
> As I already mentioned in the bug report linked above, I'm not
> quite convinced that's the case, and I don't see why we wouldn't
> just use the options and basically ignore the QEMU-level devices,
> as the former approach would work on old QEMU releases as well as
> recent ones with no drawback I can think of.
The QEMU maintainers were against the idea of us doing that.
I don't recall any QEMU developer specifically saying that, but
that might be just a case of my memory sucking :) CC'ing Eduardo
so he can weigh in.
In the
future they may add properties to, or change the defaults on, the
-transitional or -non-transitional devices only, associated with
new machine type versions. If libvirt forever uses the old devices,
then we loose ability to take advantage of that.
Regardless of what libvirt ends up doing, from the QEMU user point
of view I think it would be very surprising if eg. virtio-blk-pci
plugged into a PCIe slot behaved differently from
virtio-blk-pci-non-transitional plugged into the very same slot, or
if virtio-net-pci,disable-legacy=false,disable-modern=false behaved
differently from virtio-net-pci-transitional regardless of the slot
it's plugged into, so moving away from that consistency should be a
non-goal IMHO.
Indeed if QEMU maintainers wanted us to use the
disable-legacy/modern
features long term, there would be no point in them even adding the
new device types in the first place.
Yeah, after commenting on the bug report mentioned above I indeed
started thinking that we could have gotten away with not adding
those devices. They might still be useful to people running QEMU
directly, though.
We should only ever use the disable- flags if the new devices do
not exist in QEMU.
Wouldn't that potentially cause issues when migrating from QEMU
< 4.0.0, where we'd use disable-*, to QEMU >= 4.0.0, where we'd
use *-{,non}transitional instead? I guess not if the changes in
device behavior are gated by the machine type version.
--
Andrea Bolognani / Red Hat / Virtualization