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

["Alex Bennée" <alex.bennee@linaro.org>]

Daniel P. Berrangé <berrange@redhat.com> writes:

> 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.

We also deprecate things that stop us moving the code forward. c.f. the
long process to deprecate 32 bit hosts.

> 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.

As far as I'm aware BE Arm is a very small niche and I'm not even sure
anyone runs BE Arm systems with VirtIO - let alone legacy VirtIO. If
there are people that need this functionality they need to at least make
themselves known.

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro