Re: [Qemu-devel] [PATCH v8 3/5] Introduce irqchip type specification for KVM

[Christoffer Dall]

On Wed, Aug 12, 2015 at 2:59 PM, Peter Maydell <address@hidden> wrote:
> On 12 August 2015 at 13:27, Pavel Fedin <address@hidden> wrote:
>>  Hello!
>>
>>> I still think this is the wrong approach -- see my remarks
>>> in the previous round of patch review.
>>
>>  You know... I thought a little bit...
>>  So far, test = true in KVM_CREATE_DEVICE means that we just want to know 
>> whether this type is supported. No actual actions is done by the kernel. Is 
>> it correct? If yes, we can just leave this test as it is, because if it says 
>> that GICv2 is supported by KVM_CREATE_DEVICE, then:
>> 1. We use new API. No KVM_IRQCHIP_CREATE.
>> 2. GICv3 may be supported.
>>
>>  Therefore, if we do this check, and it succeeds, then we just
>> proceed, and later actually try to create GICv3. If it fails for some
>> reason, we will see error message anyway. So would it be OK just not
>> to touch kvm_arch_irqchip_create() at all?
>
> No, because then if the kernel only supports GICv3 the
> code in kvm_arch_irqchip_create() (as it is currently written)
> will erroneously fall back to using the old API.
>
> Christoffer: the question was, why does kvm_arch_irqchip_create()
> not only check the KVM_CAP_DEVICE_CTRL but also try to see
> if it can KVM_CREATE_DEVICE the GICv2 in order to avoid
> falling back to the old pre-KVM_CREATE_DEVICE API ? Are there
> kernels which have the capability bit set but which can't
> actually use KVM_CREATE_DEVICE to create the irqchip?

My thinking probably was that technically the KVM_CAP_DEVICE_CTRL is
an orthogonal concept from how to create the vgic, and you could
advertise this capability without also supporting the GICv2 device
type.

However, I don't believe such kernels exist and they cannot in the
future as that would be because we would remove an actively supported
API.

>
> My preference here would be for kvm_arch_irqchip_create()
> to just use 'is the KVM_CAP_DEVICE_CTRL capability set'
> for its "can we use the new API" test; that will then
> work whether we have a GICv2 or GICv3 in the host. (The
> actual GIC device creation later on might fail, of course,
> but that can be handled at that point. The only thing
> we need to do as early as kvm_arch_irqchip_create is
> determine whether we must use the old API.)
>
I'm fine with this.

-Christoffer