[PATCH v4 04/14] KVM: x86: Do not advertise SME, VM_PAGE_FLUSH, or unknown features

[Sean Christopherson <seanjc@google.com>]

Add a reverse-CPUID entry for the memory encryption word, 0x8000001F.EAX,
and use it to override the supported CPUID flags reported to userspace.
Masking the reported CPUID flags avoids over-reporting KVM support, e.g.
without the mask a SEV-SNP capable CPU may incorrectly advertise SNP
support to userspace.

Explicitly omit SME and VM_PAGE_FLUSH, which are used by KVM, but not
exposed to the guest, e.g. guest access to related MSRs will fault.
Continue advertising SEV and SEV-ES, which guests of the associated type
may expect to see present, as well as SME_COHERENT, which lets the guest
know it can skip CLFLUSH operations.

Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/cpuid.c | 6 ++++++
 arch/x86/kvm/cpuid.h | 1 +
 2 files changed, 7 insertions(+)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 6bd2f8b830e4..45745c6c2161 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -514,6 +514,10 @@ void kvm_set_cpu_caps(void)
 	 */
 	kvm_cpu_cap_mask(CPUID_8000_000A_EDX, 0);
 
+	kvm_cpu_cap_mask(CPUID_8000_001F_EAX,
+		0 /* SME */ | F(SEV) | 0 /* VM_PAGE_FLUSH */ | F(SEV_ES) |
+		F(SME_COHERENT));
+
 	kvm_cpu_cap_mask(CPUID_C000_0001_EDX,
 		F(XSTORE) | F(XSTORE_EN) | F(XCRYPT) | F(XCRYPT_EN) |
 		F(ACE2) | F(ACE2_EN) | F(PHE) | F(PHE_EN) |
@@ -871,6 +875,8 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
 	case 0x8000001F:
 		if (!boot_cpu_has(X86_FEATURE_SEV))
 			entry->eax = entry->ebx = entry->ecx = entry->edx = 0;
+		else
+			cpuid_entry_override(entry, CPUID_8000_001F_EAX);
 		break;
 	/*Add support for Centaur's CPUID instruction*/
 	case 0xC0000000:
diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
index 2a0c5064497f..b3042ac6b5dc 100644
--- a/arch/x86/kvm/cpuid.h
+++ b/arch/x86/kvm/cpuid.h
@@ -80,6 +80,7 @@ static const struct cpuid_reg reverse_cpuid[] = {
 	[CPUID_8000_0007_EBX] = {0x80000007, 0, CPUID_EBX},
 	[CPUID_7_EDX]         = {         7, 0, CPUID_EDX},
 	[CPUID_7_1_EAX]       = {         7, 1, CPUID_EAX},
+	[CPUID_8000_001F_EAX] = {0x8000001f, 1, CPUID_EAX},
 };
 
 /*
-- 
2.30.1.766.gb4fecdf3b7-goog