Re: [PATCH v4] xen/arm: Allow QEMU platform to be built with GICv2

[Julien Grall <julien@xen.org>]

Hello,

On 18/01/2022 12:58, Dongjiu Geng wrote:
> Bertrand Marquis <Bertrand.Marquis@arm.com> 于2022年1月18日周二 16:48写道：
>>
>> Hi Dongju,
>>
>>> On 18 Jan 2022, at 08:45, Dongjiu Geng <gengdongjiu1@gmail.com> wrote:
>>>
>>> Julien Grall <julien@xen.org> 于2022年1月17日周一 22:16写道：
>>>>
>>>> Hi,
>>>>
>>>> On 17/01/2022 10:40, Dongjiu Geng wrote:
>>>>> It turns out that QEMU has been supporting GICv2 virtualization since
>>>>> v3.1.0. So remove the dependencies on GICv3.
>>>>
>>>>
>>>> Technically, the current form of CONFIG_QEMU allows the same binary to
>>>> boot on QEMU with GICv2 or GICv3.
>>>>
>>>>> If we want to use GICv3,
>>>>> we can select the QEMU_LEGACY configuration.
>>>>
>>>> AFAIK, GICv3 is not a legacy feature... So it feels a bit odd to name it
>>>> like that (see more below).
>>>
>>> Legacy means QEMU platform only supports GICV3, now it can support
>>> both GICv2 and GICv3. The scope of support has been expanded
>>> Not mean GICv3 is a legacy feature.
>>
>> You might be misleading a bit here.
>> In the current configuration, Xen support GICv2, GICv3 and vgic.
>> The only thing not supported is actually the new VGIC but this is an unsupported feature not fully functional which shall be used with caution.
>>
>> What issue exactly do you have in Qemu configured for gicv2 when you use the default configuration ?
> 
> I want to use NEW_VGIC with GICv2, but QEMU only select GICV3,  when
> GICv3 is select, the NEW_VGIC can not be used.   I try the NEW_VGIC
> with GICv2, not found issue. so I want to remove this limitation.
> If  you think we should not support NEW_VGIC feature,  we can ignore
> this patch.  thanks!

I would love to get the NEW_VGIC supported. But I think this is 
orthogonal to whether we want to allow CONFIG_QEMU to select it.

If we fully support the new vGIC (i.e vGICv3 is implemented) then it 
would be possible to select it with CONFIG_QEMU as the restriction 
(!GICV3) would not be present.

So I view the change here as temporary. The proposed change will have an 
impact on the existing users (i.e. CONFIG_QEMU will not work out of the 
box anymore when QEMU is configured with GICv3). In addition to that, 
the end solution doesn't look simple. So I think this is not a patch I 
am willing to see temporarily.

One possibility would be to create a new CONFIG for allowing to select 
NEW_VGIC. That said, I don't much like it. So I would say if you want to 
select NEW_VGIC then you want to use defconfig (which should contain 
enough to boot on QEMU with DT).

Cheers,

-- 
Julien Grall