[PATCH 3.16 133/245] kvm: x86: Host feature SSBD doesn't imply guest feature SPEC_CTRL_SSBD

[Ben Hutchings]

3.16.83-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Jim Mattson <jmattson@xxxxxxxxxx>

commit 396d2e878f92ec108e4293f1c77ea3bc90b414ff upstream.

The host reports support for the synthetic feature X86_FEATURE_SSBD
when any of the three following hardware features are set:
  CPUID.(EAX=7,ECX=0):EDX.SSBD[bit 31]
  CPUID.80000008H:EBX.AMD_SSBD[bit 24]
  CPUID.80000008H:EBX.VIRT_SSBD[bit 25]

Either of the first two hardware features implies the existence of the
IA32_SPEC_CTRL MSR, but CPUID.80000008H:EBX.VIRT_SSBD[bit 25] does
not. Therefore, CPUID.(EAX=7,ECX=0):EDX.SSBD[bit 31] should only be
set in the guest if CPUID.(EAX=7,ECX=0):EDX.SSBD[bit 31] or
CPUID.80000008H:EBX.AMD_SSBD[bit 24] is set on the host.

Fixes: 0c54914d0c52a ("KVM: x86: use Intel speculation bugs and features as derived in generic x86 code")
Signed-off-by: Jim Mattson <jmattson@xxxxxxxxxx>
Reviewed-by: Jacob Xu <jacobhxu@xxxxxxxxxx>
Reviewed-by: Peter Shier <pshier@xxxxxxxxxx>
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Reported-by: Eric Biggers <ebiggers@xxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
[bwh: Backported to 3.16: adjust indentation]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 arch/x86/kvm/cpuid.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -400,7 +400,8 @@ static inline int __do_cpuid_ent(struct
 				entry->edx |= F(SPEC_CTRL);
 			if (boot_cpu_has(X86_FEATURE_STIBP))
 				entry->edx |= F(INTEL_STIBP);
-			if (boot_cpu_has(X86_FEATURE_SSBD))
+			if (boot_cpu_has(X86_FEATURE_SPEC_CTRL_SSBD) ||
+			    boot_cpu_has(X86_FEATURE_AMD_SSBD))
 				entry->edx |= F(SPEC_CTRL_SSBD);
 			/*
 			 * We emulate ARCH_CAPABILITIES in software even