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

[Bertrand Marquis <Bertrand.Marquis@arm.com>]

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 ?

Cheers
Bertrand

> 
>> 
>>> 
>>> Signed-off-by: Dongjiu Geng <gengdongjiu1@gmail.com>
>>> ---
>>>  xen/arch/arm/platforms/Kconfig | 10 +++++++++-
>>>  1 file changed, 9 insertions(+), 1 deletion(-)
>>> 
>>> diff --git a/xen/arch/arm/platforms/Kconfig b/xen/arch/arm/platforms/Kconfig
>>> index c93a6b2756..41e82a42ee 100644
>>> --- a/xen/arch/arm/platforms/Kconfig
>>> +++ b/xen/arch/arm/platforms/Kconfig
>>> @@ -13,7 +13,15 @@ config ALL_PLAT
>>>      automatically select any of the related drivers.
>>> 
>>>  config QEMU
>>> -     bool "QEMU aarch virt machine support"
>>> +     bool "QEMU aarch virt machine support >= v3.1.0"
>> 
>> This is a bit misleading. A user may select this thinking that this will
>> select GICv3. However, this will not.
>> 
>> This also raises the question of what is the default GIC version in QEMU
>> (i.e. if you don't pass anything on the command line)? If this is GICv3,
>> then I am afraid that this patch would be a no-go for me.
>> 
>> Looking at overall discussion, you seem to push the patch only to allow
>> building a tiny Xen for QEMU and the new vGIC.
>> 
>> The default Xen (i.e. make defconfig) will also work on QEMU. Given that
>> the new vGIC is a still in development, I am seriously considering to
>> say that if you want to try it then you have to use the default
>> configuration.
>> 
>> @Dongjiu, is there any reason why you want to use the tiny QEMU config
>> rather than the default configuration?
> 
> Hi Julien, thanks for the reply, I do not use the tiny QEMU config. I
> used the default configuration and selected the platform as QEMU.
> But QEMU platform only slects GICV3, so I submit this patch to remove
> the limitation because I want to use VGIC. VGIC can already support
> GICv2 now.
> 
>> 
>> @Bertrand, @Stefano, what do you think?
>> 
>> Cheers,
>> 
>> --
>> Julien Grall