x86/cpufeatures: Add the Virtual SPEC_CTRL feature

Newer AMD processors have a feature to virtualize the use of the
SPEC_CTRL MSR. Presence of this feature is indicated via CPUID
function 0x8000000A_EDX[20]: GuestSpecCtrl. When present, the
SPEC_CTRL MSR is automatically virtualized.

Signed-off-by: Babu Moger <babu.moger@amd.com>
Acked-by: Borislav Petkov <bp@suse.de>
Message-Id: <161188100272.28787.4097272856384825024.stgit@bmoger-ubuntu>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Author: babumoger

arch/x86/include/asm/cpufeatures.h

@@ -336,6 +336,7 @@
 #define X86_FEATURE_AVIC		(15*32+13) /* Virtual Interrupt Controller */
 #define X86_FEATURE_V_VMSAVE_VMLOAD	(15*32+15) /* Virtual VMSAVE VMLOAD */
 #define X86_FEATURE_VGIF		(15*32+16) /* Virtual GIF */
+#define X86_FEATURE_V_SPEC_CTRL		(15*32+20) /* Virtual SPEC_CTRL */
 #define X86_FEATURE_SVME_ADDR_CHK	(15*32+28) /* "" SVME addr check */
 
 /* Intel-defined CPU features, CPUID level 0x00000007:0 (ECX), word 16 */