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

[Peter Maydell]

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.

> +
> +VM migration is not guaranteed when using ``-cpu max``, as features supported
> +may change between QEMU versions. To ensure your VM can be migrated, it is
> +recommended to use another cpu model instead.

This paragraph is good, though -- that 'max' doesn't work for migration
is important, and we should definitely document that.

thanks
-- PMM