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



On Wed, Jan 16, 2019 at 12:45:43PM -0200, Eduardo Habkost wrote:
> On Wed, Jan 16, 2019 at 02:37:22PM +0000, Daniel P. Berrangé wrote:
> > On Wed, Jan 16, 2019 at 03:31:49PM +0100, Andrea Bolognani wrote:
> > > On Wed, 2019-01-16 at 12:44 +0000, Daniel P. Berrangé wrote:
> > > > 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:
> > > > > > 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.
> > > > > 
> > > [...]
> > > > 
> > > > 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.
> > > 
> > > Well, let's see if Eduardo is willing to reconsider the policy on
> > > compatibility between virtio-*-pci-{,non-}transitional and plain
> > > virtio-*-pci going forward based on the principle-of-least-surprise
> > > rationale outlined above :)
> > 
> > I think we should use the new devices no matter what. Libvirt generally
> > always strives to follow the latest QEMU best practice, even when new &
> > old way of doing something are functionally identical. Eventually we
> > would drop support for QEU < 4.0 and the old way would go away entirely.
> 
> It would also allow us to deprecate the old devices, which would
> be welcome.  Always using the new devices when available would be
> my recommendation.

I don't really see QEMU upstream deprecating the old devices any
time. There is sooooo much documentation that refers to them that
will never be fixed. 99% of users won't get any benefit from using
the new devices either, so there's no compelling reason to update
their existing configs or docs. They're not going to be a huge maint
burden to QEMU devs either given its just a toggle of a few props.

I might see a downstream distro deprecating them at some point
though, since they have a much tighter controlled usage scenario
than upstream.

> But I don't want to create unnecessary obstacles for libvirt, so
> if there's a real benefit in promising compatibility between both
> device types, we can still promise that on the QEMU side.

I don't think there's an obstacle for libvirt, as I don't see any
compelling reason to avoid the new devices when we have QEMU >= 4.0.

> Breaking compatibility on purpose is very unlikely, and the most
> likely accidents could be detected by
> tests/acceptance/virtio_version.py.

Regards,
Daniel
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