Re: [PATCH] KVM: x86: Advertise SUCCOR and OVERFLOW_RECOV cpuid bits

[Sean Christopherson <seanjc@google.com>]

On Tue, Jul 30, 2024, John Allen wrote:
> Handling deferred, uncorrected MCEs on AMD guests is now possible with
> additional support in qemu. Ensure that the SUCCOR and OVERFLOW_RECOV
> bits are advertised to the guest in KVM.
> 
> Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: John Allen <john.allen@amd.com>
> ---
>  arch/x86/kvm/cpuid.c   | 2 +-
>  arch/x86/kvm/svm/svm.c | 7 +++++++
>  2 files changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 2617be544480..4745098416c3 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -1241,7 +1241,7 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
>  
>  		/* mask against host */
>  		entry->edx &= boot_cpu_data.x86_power;
> -		entry->eax = entry->ebx = entry->ecx = 0;
> +		entry->eax = entry->ecx = 0;

Needs an override to prevent reporting all of EBX to userspace.

		cpuid_entry_override(entry, CPUID_8000_0007_EBX);

>  		break;
>  	case 0x80000008: {
>  		/*
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index c115d26844f7..a6820b0915db 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -5199,6 +5199,13 @@ static __init void svm_set_cpu_caps(void)
>  		kvm_cpu_cap_set(X86_FEATURE_SVME_ADDR_CHK);
>  	}
>  
> +	/* CPUID 0x80000007 */
> +	if (boot_cpu_has(X86_FEATURE_SUCCOR))
> +		kvm_cpu_cap_set(X86_FEATURE_SUCCOR);
> +
> +	if (boot_cpu_has(X86_FEATURE_OVERFLOW_RECOV))
> +		kvm_cpu_cap_set(X86_FEATURE_OVERFLOW_RECOV);

This _could_ use kvm_cpu_cap_check_and_set(), but given that this an AMD specific
leaf and unlikely to ever be used by Intel, I'm inclined to handle this in cpuid.c,
with an opporunustic "conversion" to one feature per line[*]:

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 2617be544480..ea11a7e45174 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -743,6 +743,11 @@ void kvm_set_cpu_caps(void)
        if (!tdp_enabled && IS_ENABLED(CONFIG_X86_64))
                kvm_cpu_cap_set(X86_FEATURE_GBPAGES);
 
+       kvm_cpu_cap_mask(CPUID_8000_0007_EBX,
+               F(OVERFLOW_RECOV) |
+               F(SUCCOR)
+       );
+
        kvm_cpu_cap_init_kvm_defined(CPUID_8000_0007_EDX,
                SF(CONSTANT_TSC)
        );


[*] https://lore.kernel.org/all/ZoxooTvO5vIEnS5V@google.com