James,

On 2017/9/1 2:04, James Morse wrote:

Hi Dongjiu Geng,

On 28/08/17 11:38, Dongjiu Geng wrote:

quoted

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...

Ok, thanks.

quoted

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.

quoted

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.

shenglong (zhaoshenglong at huawei.com) who is Qemu maintainer suggested checking the CPU RAS-extensions
to decide whether generate the APEI table and record CPER for the guest OS in the user space.
he means if the host does not support RAS, user space may also not support RAS.

quoted

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'

I understand your suggestion.

quoted

+		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


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