Re: [PATCH v2 2/2] docs/about/deprecated.rst: document new restrictions for isapc CPU models

[Mark Cave-Ayland <mark.caveayland@nutanix.com>]

On 26/09/2025 13:05, Peter Maydell wrote:

> On Thu, 25 Sept 2025 at 17:07, Mark Cave-Ayland
> <mark.caveayland@nutanix.com> wrote:
>>
>> Add a new paragraph in the "Backwards compatibility" section documenting that
>> the isapc machine is now restricted to 32-bit x86 CPUs, and -cpu host and
>> -cpu max are no longer supported.
>>
>> Signed-off-by: Mark Cave-Ayland <mark.caveayland@nutanix.com>
>> ---
>>   docs/about/deprecated.rst | 13 +++++++++++++
>>   1 file changed, 13 insertions(+)
>>
>> diff --git a/docs/about/deprecated.rst b/docs/about/deprecated.rst
>> index aa300bbd50..4c7000650d 100644
>> --- a/docs/about/deprecated.rst
>> +++ b/docs/about/deprecated.rst
>> @@ -514,6 +514,19 @@ available firmwares that are using the current (wrong) name.  The
>>   property is kept as is in 9.1, together with "riscv,delegation", to
>>   give more time for firmware developers to change their code.
>>
>> +x86 "isapc" board restricted to 32-bit x86 CPUs (since 10.2)
>> +''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
>> +
>> +The "isapc" board represents a historical x86 ISA PC and is intended for
>> +older 32-bit x86 CPU models, defaulting to a 486 CPU model.  Previously it
>> +was possible (but non-sensical) to specify a more modern x86 CPU, including
>> +``-cpu host`` or ``-cpu max`` even if the features were incompatible with many
>> +of the intended guest OSs.
>> +
>> +Now the "isapc" board contains an explicit list of supported 32-bit x86 CPU
>> +models. If the user requests an unsupported CPU model then an error message is
>> +returned indicating the available CPU models.
> 
> This should go in "removed.rst", I think. "deprecated.rst" is for
> things that still work today (perhaps with a warning) but will stop
> working in a future QEMU release, not for things that we have already
> caused to stop working. (And we should only go directly to
> 'removed' for features that we are sure nobody's using, but
> I assume we've already had that discussion for the code change.)

It's an interesting one because rather than removing something, we're 
adding a validation check to prevent the user from doing something that 
is nonsensical, i.e. using a modern CPU on a board/OSs that would never 
happen in real life. Given that just about everyone other than hobbyists 
is using the pc/q35 machines on x86, the target audience for isapc is 
comparatively small.

There was some concern that libvirt could generate -cpu host/-cpu max 
for isapc on the command line which is why I added the original 
workaround, but then Igor intervened as maintainer and requested this to 
be removed and enforced via valid_cpu_types instead.

Where do you think would be a suitable place in removed.rst? Maybe the 
"System emulator machines" section?


ATB,

Mark.