Re: [PATCH] hw/arm: Deprecate legacy VirtIO devices on big-endian guests

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Dec 17, 2025 at 03:06:57PM +0100, Philippe Mathieu-Daudé wrote:
> We couldn't find a way (guest OS with VirtIO drivers) to test
> a legacy VirtIO device on a ARM vCPU running in big-endian.
> 
> Deprecate for the v11.0 release, giving 1 year to users who
> really care to contribute functional tests.
> 
> Suggested-by: Alex Bennée <alex.bennee@linaro.org>
> Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
> ---
>  docs/about/deprecated.rst | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/docs/about/deprecated.rst b/docs/about/deprecated.rst
> index ac31a2bce42..3a69facb0f1 100644
> --- a/docs/about/deprecated.rst
> +++ b/docs/about/deprecated.rst
> @@ -515,6 +515,17 @@ It was implemented as a no-op instruction in TCG up to QEMU 9.0, but
>  only with ``-cpu max`` (which does not guarantee migration compatibility
>  across versions).
>  
> +VirtIO devices
> +''''''''''''''
> +
> +Legacy VirtIO devices on Big-Endian ARM architecture (since 11.0)
> +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> +
> +There are no functional tests for legacy virtio devices used by ARM
> +machines running in big-endian order, which makes harder to maintain
> +the code path while the code base evolve.

Lack of test coverage is not a reason to deprecate something.

We deprecate things we intend to intentionally remove or intentionally
change in an incompatible manner.

If something is not tested, that merely means it has lesser quality
guarantees, and is liable to unintenionally get broken at times.

If we're planning to *intentionally*  remove the ability to use
legacy virtio on big endian, that would be a reason to deprecate.
If so the deprecation message should say this, not talk about
missing functional testing.

With regards,
Daniel
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