linux-kernel - [PATCH] KVM: x86: skip host CPUID call for hypervisor leaves

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Date:   Thu, 20 Jan 2022 12:50:15 -0500
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: [PATCH] KVM: x86: skip host CPUID call for hypervisor leaves

Hypervisor leaves are always synthesized by __do_cpuid_func.  Just return
zeroes and do not ask the host, it would return a bogus value anyway if
it were used.

Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/kvm/cpuid.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 3902c28fb6cb..fd949e89120a 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -692,9 +692,17 @@ static struct kvm_cpuid_entry2 *do_host_cpuid(struct kvm_cpuid_array *array,
 
 	entry = &array->entries[array->nent++];
 
+	memset(entry, 0, sizeof(*entry));
 	entry->function = function;
 	entry->index = index;
-	entry->flags = 0;
+	switch (function & 0xC0000000) {
+	case 0x40000000:
+		/* Hypervisor leaves are always synthesized by __do_cpuid_func.  */
+		return entry;
+
+	default:
+		break;
+	}
 
 	cpuid_count(entry->function, entry->index,
 		    &entry->eax, &entry->ebx, &entry->ecx, &entry->edx);
-- 
2.31.1