Re: [PATCH v2 3/3] docs/system/arm/virt: mention specific migration information

[Thomas Huth]

On 10/01/2025 21.54, Pierrick Bouvier wrote:
> On 1/10/25 08:30, Peter Maydell wrote:
> > On Thu, 19 Dec 2024 at 18:32, Pierrick Bouvier
> > <pierrick.bouvier@linaro.org> wrote:
> > > Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
> > > ---
> > >   docs/system/arm/virt.rst | 14 +++++++++++---
> > >   1 file changed, 11 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/docs/system/arm/virt.rst b/docs/system/arm/virt.rst
> > > index d25275c27ce..9f1457cf9a2 100644
> > > --- a/docs/system/arm/virt.rst
> > > +++ b/docs/system/arm/virt.rst
> > > @@ -17,9 +17,17 @@ to have the same behaviour as that of previous QEMU 
> > > releases, so
> > >   that VM migration will work between QEMU versions. For instance the
> > >   ``virt-5.0`` machine type will behave like the ``virt`` machine from
> > >   the QEMU 5.0 release, and migration should work between ``virt-5.0``
> > > -of the 5.0 release and ``virt-5.0`` of the 5.1 release. Migration
> > > -is not guaranteed to work between different QEMU releases for
> > > -the non-versioned ``virt`` machine type.
> > > +of the 5.0 release and ``virt-5.0`` of the 5.1 release.
> > > +
> > > +When saving a VM using the ``virt`` model, the snapshot is automatically 
> > > set to
> > > +target the latest ``virt`` versioned model. When loading the VM with a more
> > > +recent QEMU version, you'll need to set machine model to match the 
> > > version of
> > > +your snapshot. When loading it, QEMU will return an error with the expected
> > > +``virt`` version you should set, so you don't need to record it.
> >
> > I don't think we should be encouraging this -- our standard approach
> > is "use the versioned machine types if you want migration", not
> > "you can start with an unversioned type on the source end". So I've
> > dropped this paragraph.
>
> That's fine for me, I don't have a strong opinion on this.
> I just had a (good) surprise when I saved a vm with virt machine, and 
> realised it's versioned by default. It's good to know that when you export a 
> virt machine, you are guaranteed it's bound to a specific version, so you 
> can always load it with new QEMU versions. This is what I tried to express 
> with this paragraph.

Technically, the "virt" machine is not a real machine, but an alias of the 
latest machine:

$ ./qemu-system-aarch64 -M help | grep alias
virt                 QEMU 10.0 ARM Virtual Machine (alias of virt-10.0)

So maybe that could be mentioned here instead?

 Thomas