linux-kernel - [PATCH] KVM: x86: Do not expose the host value of CPUID.8000001EH

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Message-Id: <20221022082643.1725875-1-pbonzini@redhat.com>
Date:   Sat, 22 Oct 2022 04:26:43 -0400
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     seanjc@...gle.com, jmattson@...gle.com
Subject: [PATCH] KVM: x86: Do not expose the host value of CPUID.8000001EH

Several fields of CPUID.8000001EH (ExtendedApicId in EAX[31:0],
CoreId in EBX[7:0], NodeId in ECX[7:0]) vary on each processor,
and it is simply impossible to fit the right values in the
KVM_GET_SUPPORTED_CPUID API, in such a way that they can be
passed to KVM_SET_CPUID2.

The most likely way to avoid confusion in the guest is to zero
out all the values.  Userspace will most likely override it
anyway if it want to present a specific topology to the guest.

This patch essentially reverts commit 382409b4c43e ("kvm: x86: Include
CPUID leaf 0x8000001e in kvm's supported CPUID").

Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/kvm/cpuid.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index a0292ba650df..380b71600a9e 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -1193,6 +1193,9 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
 		entry->ebx = entry->ecx = entry->edx = 0;
 		break;
 	case 0x8000001e:
+		/* Different on each processor, just hide it.  */
+		entry->eax = entry->ebx = entry->ecx = 0;
+		entry->edx = 0;
 		break;
 	case 0x8000001F:
 		if (!kvm_cpu_cap_has(X86_FEATURE_SEV)) {
-- 
2.31.1