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 12:56 PM, Mohamed Mediouni wrote:
> 
> 
>> On 4. Aug 2025, at 21:50, Pierrick Bouvier 
>> <pierrick.bouvier@linaro.org> wrote:
>>
>> On 8/4/25 7:23 AM, Mohamed Mediouni wrote:
>>> Windows Hypervisor Platform's vGIC doesn't support ITS.
>>> Deal with this by reporting to the user and not creating the ITS device.
>>> Signed-off-by: Mohamed Mediouni <mohamed@unpredictable.fr>
>>> ---
>>>  hw/arm/virt.c | 10 ++++++++++
>>>  1 file changed, 10 insertions(+)
>>> diff --git a/hw/arm/virt.c b/hw/arm/virt.c
>>> index 98a1c74c42..0039f6a12b 100644
>>> --- a/hw/arm/virt.c
>>> +++ b/hw/arm/virt.c
>>> @@ -741,6 +741,16 @@ static void create_its(VirtMachineState *vms)
>>>          return;
>>>      }
>>>  +    if (whpx_enabled() && vms->tcg_its) {
>>> +        /*
>>> +         * Signal to the user when ITS is neither supported by the host
>>> +         * nor emulated by the machine.
>>> +         */
>>> +        info_report("ITS not supported on WHPX.");
>>> +        info_report("To support MSIs, use its=off to enable GICv3 + 
>>> GICv2m.");
>>> +        return;
>>> +    }
>>> +
>>>      dev = qdev_new(its_class_name());
>>>        object_property_set_link(OBJECT(dev), "parent-gicv3", 
>>> OBJECT(vms->gic),
>>
>> 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?

Thanks,
Pierrick