[PATCH] KVM: x86: advertise absence of X86_BUG_NULL_SEG via CPUID

[Paolo Bonzini]

Guests have X86_BUG_NULL_SEG if and only if the host have it.  Use
the info from static_cpu_has_bug to form the 0x80000021 CPUID leaf that
was defined for Zen3.  Userspace can then set the bit even on older
CPUs that do not have the bug, such as Zen2.

Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
 arch/x86/kvm/cpuid.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 2d70edb0f323..b51398e1727b 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -902,7 +902,13 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
 		entry->edx = 0;
 		break;
 	case 0x80000000:
-		entry->eax = min(entry->eax, 0x8000001f);
+		entry->eax = min(entry->eax, 0x80000021);
+		/*
+		 * X86_BUG_NULL_SEG is not reported in CPUID on Zen2; in
+		 * that case, provide the CPUID leaf ourselves.
+		 */
+		if (!static_cpu_has_bug(X86_BUG_NULL_SEG))
+			entry->eax = max(entry->eax, 0x80000021);
 		break;
 	case 0x80000001:
 		cpuid_entry_override(entry, CPUID_8000_0001_EDX);
@@ -973,6 +979,15 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
 			entry->ebx &= ~GENMASK(11, 6);
 		}
 		break;
+	case 0x80000020:
+		entry->eax = entry->ebx = entry->ecx = entry->edx = 0;
+		break;
+	case 0x80000021:
+		entry->ebx = entry->ecx = entry->edx = 0;
+		entry->eax &= BIT(6);
+		if (!static_cpu_has_bug(X86_BUG_NULL_SEG))
+			entry->eax |= BIT(6);
+		break;
 	/*Add support for Centaur's CPUID instruction*/
 	case 0xC0000000:
 		/*Just support up to 0xC0000004 now*/
-- 
2.27.0