Re: [PATCH 2/3] KVM: x86: always expose VIRT_SSBD to guests

[Eduardo Habkost <ehabkost@redhat.com>]

On Wed, Aug 21, 2019 at 10:26:41AM +0200, Paolo Bonzini wrote:
> Even though it is preferrable to use SPEC_CTRL (represented by
> X86_FEATURE_AMD_SSBD) instead of VIRT_SPEC, VIRT_SPEC is always
> supported anyway because otherwise it would be impossible to
> migrate from old to new CPUs.  Make this apparent in the
> result of KVM_GET_SUPPORTED_CPUID as well.
> 
> While at it, reuse X86_FEATURE_* constants for the SVM leaf too.
> 
> However, we need to hide the bit on Intel processors, so move
> the setting to svm_set_supported_cpuid.
> 
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> Reported-by: Eduardo Habkost <ehabkost@redhat.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
[...]
> @@ -5944,6 +5944,11 @@ static void svm_set_supported_cpuid(u32 func, struct kvm_cpuid_entry2 *entry)
>  		if (nested)
>  			entry->ecx |= (1 << 2); /* Set SVM bit */
>  		break;
> +	case 0x80000008:
> +		if (boot_cpu_has(X86_FEATURE_LS_CFG_SSBD) ||
> +		     boot_cpu_has(X86_FEATURE_AMD_SSBD))
> +			entry->ebx |= F(VIRT_SSBD);
> +		break;

Wasn't the old code at arch/x86/kvm/cpuid.c:__do_cpuid_func()
supposed to be deleted?

               /*
                * The preference is to use SPEC CTRL MSR instead of the
                * VIRT_SPEC MSR.
                */
               if (boot_cpu_has(X86_FEATURE_LS_CFG_SSBD) &&
                   !boot_cpu_has(X86_FEATURE_AMD_SSBD))
                       entry->ebx |= F(VIRT_SSBD);


-- 
Eduardo