Re: [PATCH v4 12/17] hw/arm: virt: cleanly fail on attempt to use the platform vGIC together with ITS

[Pierrick Bouvier <pierrick.bouvier@linaro.org>]

On 8/4/25 1:59 PM, Mohamed Mediouni wrote:
> 
> 
>> On 4. Aug 2025, at 22:40, Pierrick Bouvier <pierrick.bouvier@linaro.org> wrote:
>>
>> On 8/4/25 12:56 PM, Mohamed Mediouni wrote:
>>>> On 4. Aug 2025, at 21:50, Pierrick Bouvier <pierrick.bouvier@linaro.org> wrote:
>>>>
>>>> Is it equivalent to simply using its=off, or is there a difference?
>>>> The info_report seems to imply it's not the same.
>>> Not equivalent.
>>> Regular system: GICv3 + ITS
>>> This configuration (for the newest machine version): GICv3 with no MSIs
>>> And its=off explicitly: GICv3 + GICv2m
>>> It became not equivalent since the intro of GICv3 + GICv2m in patch 2 of this series.
>>>
>>> Thank you,
>>
>> I see. It could be worth adding this information to commit message.
>> With that,
>> Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
>>
>> Slightly off topic for this commit, is there any downside to always have GICv3 + GICv2m setup enabled? Do some systems don't support GICv2m?
>>
> GICv2m is modelled as a device external from the GIC, and so it can be emulated everywhere.
> 
> It can be done but then it supposes that the same Qemu command line will expose a different device model (instead of just MSI support missing) when ran on a different system. Not sure that’s the right thing to do…
> 

Indeed, it might be surprising, but on the other side, we already do 
something specific to whpx, so it might be better to choose the default 
offering the maximum support.

 From user point of view, the question is how hard it would be for them 
to guess what is missing in case they need MSI support, and didn't see 
the warning message about using its=off in this case.

Peter, any opinion on this?