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


  • To: Dongjiu Geng <gengdongjiu1@xxxxxxxxx>
  • From: Bertrand Marquis <Bertrand.Marquis@xxxxxxx>
  • Date: Tue, 18 Jan 2022 09:17:42 +0000
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  • Cc: Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Tue, 18 Jan 2022 09:18:09 +0000
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  • Thread-topic: [PATCH v4] xen/arm: Allow QEMU platform to be built with GICv2

Hi Dongju,

> On 18 Jan 2022, at 08:58, Dongjiu Geng <gengdongjiu1@xxxxxxxxx> wrote:
> 
> Bertrand Marquis <Bertrand.Marquis@xxxxxxx> 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@xxxxxxxxx> wrote:
>>> 
>>> Julien Grall <julien@xxxxxxx> 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!

Supporting GICv2 makes sense but using NEW_VGIC in Xen might not as it is not 
security supported and does not support ITS and MSIs.

Do you have a reason to use the NEW_VGIC implementation instead of the standard 
one ?

Cheers
Bertrand

> 
>> 
>> Cheers
>> Bertrand
>> 
>>> 
>>>> 
>>>>> 
>>>>> Signed-off-by: Dongjiu Geng <gengdongjiu1@xxxxxxxxx>
>>>>> ---
>>>>> 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


 


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