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Re: [libvirt] [PATCH 4/6] qemu: Wire up disk model=virtio-{non-}transitional



On Wed, Jan 16, 2019 at 01:29:13PM +0100, Andrea Bolognani wrote:
> 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.

It was somewhere in one of the many mail threads, but I'm not
finding the archive link right now.

> > 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.

In this message Eduardo said virtio-blk-pci,disable-legacy and
virtio-blk-pci-non-transitional are only compatible with the
pc-4.0 machine types and earlier. There's no compat guarantee
of compat for future machine types:

  https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg03762.html

If we didn't use the new QEMU device models right now, we could end
up trapped forever. The safe futureproof approach is to always use
the new devices models if available, and use disable-legacy for old
QEMU versions only, which we know will have old machine types for
which the compat guarantee is available.

Regards,
Daniel
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