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

[Paolo Bonzini]

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.

Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Reported-by: Eduardo Habkost <ehabkost@xxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
 arch/x86/kvm/cpuid.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 145ec050d45d..5865bc73bbb5 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -747,11 +747,13 @@ static inline int __do_cpuid_func(struct kvm_cpuid_entry2 *entry, u32 function,
 		entry->ebx &= kvm_cpuid_8000_0008_ebx_x86_features;
 		cpuid_mask(&entry->ebx, CPUID_8000_0008_EBX);
 		/*
-		 * The preference is to use SPEC CTRL MSR instead of the
-		 * VIRT_SPEC MSR.
+		 * VIRT_SPEC is only implemented for AMD processors,
+		 * but the host could set AMD_SSBD if it wanted even
+		 * for Intel processors.
 		 */
-		if (boot_cpu_has(X86_FEATURE_LS_CFG_SSBD) &&
-		    !boot_cpu_has(X86_FEATURE_AMD_SSBD))
+		if ((boot_cpu_has(X86_FEATURE_LS_CFG_SSBD) ||
+		     boot_cpu_has(X86_FEATURE_AMD_SSBD)) &&
+		    boot_cpu_has(X86_FEATURE_SVM))
 			entry->ebx |= F(VIRT_SSBD);
 		break;
 	}
-- 
1.8.3.1