Re: [PATCH v6 4/7] arm64: kvm: support user space to query RAS extension feature

[James Morse]

Hi Dongjiu Geng,

On 28/08/17 11:38, Dongjiu Geng wrote:
> In ARMV8.2 RAS extension, a virtual SError exception syndrome
> register(VSESR_EL2) is added.  This value may be specified from
> userspace.

I agree that the CPU support for injecting SErrors with a specified ESR should
be exposed to KVM's user space...


> Userspace will want to check if the CPU has the RAS
> extension. 

... but user-space wants to know if it can inject SErrors with a specified ESR.

What if we gain another way of doing this that isn't via the RAS-extensions, now
user-space has to check for two capabilities.


> If it has, it wil specify the virtual SError syndrome
> value, otherwise it will not be set. This patch adds support for
> querying the availability of this extension.

I'm against telling user-space what features the CPU has unless it can use them
directly. In this case we are talking about a KVM API, so we should describe the
API not the CPU.


> diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
> index 3256b9228e75..b7313ee028e9 100644
> --- a/arch/arm64/kvm/reset.c
> +++ b/arch/arm64/kvm/reset.c
> @@ -77,6 +77,9 @@ int kvm_arch_dev_ioctl_check_extension(struct kvm *kvm, long ext)
>  	case KVM_CAP_ARM_PMU_V3:
>  		r = kvm_arm_support_pmu_v3();
>  		break;
> +	case KVM_CAP_ARM_RAS_EXTENSION:

This should be called something more like 'KVM_CAP_ARM_INJECT_SERROR_ESR'


> +		r = cpus_have_const_cap(ARM64_HAS_RAS_EXTN);
> +		break;
>  	case KVM_CAP_SET_GUEST_DEBUG:
>  	case KVM_CAP_VCPU_ATTRIBUTES:
>  		r = 1;


We can inject SError on systems without the RAS extensions using just the
HCR_EL2.VSE bit. We may want to make the 'ESR' part of the API optional, or
expose '1' for the without-ESR version and '2 for with-ESR, (however we choose
to implement that).

The risk is if we want to add a without-ESR version later, and the name we make
ABI now turned out to be a mistake. Marc or Christoffer probably have the best
view of this. (no-one has needed it so far...)


Thanks,

James