linux-kernel - [PATCH] KVM: x86: expose MSR_PLATFORM

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Message-ID: <20241113120145.388071-1-pbonzini@redhat.com>
Date: Wed, 13 Nov 2024 07:01:44 -0500
From: Paolo Bonzini <pbonzini@...hat.com>
To: linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Subject: [PATCH] KVM: x86: expose MSR_PLATFORM_INFO as a feature MSR

For userspace that wants to disable KVM_X86_QUIRK_STUFF_FEATURE_MSRS, it
is useful to know what bits can be set to 1 in MSR_PLATFORM_INFO (apart
from the TSC ratio).  The right way to do that is via /dev/kvm's
feature MSR mechanism.

In fact, MSR_PLATFORM_INFO is already a feature MSR for the purpose of
blocking updates after the vCPU is run, but KVM_GET_MSRS did not return
a valid value for it.

Just like in a VM that leaves KVM_X86_QUIRK_STUFF_FEATURE_MSRS enabled,
the TSC ratio field is left to 0.  Only bit 31 is set.

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

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 8637bc001096..03b409deb7d7 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1675,6 +1675,9 @@ static int kvm_get_feature_msr(struct kvm_vcpu *vcpu, u32 index, u64 *data,
 	case MSR_IA32_PERF_CAPABILITIES:
 		*data = kvm_caps.supported_perf_cap;
 		break;
+	case MSR_PLATFORM_INFO:
+		msr->data = MSR_PLATFORM_INFO_CPUID_FAULT;
+		break;
 	case MSR_IA32_UCODE_REV:
 		rdmsrl_safe(index, data);
 		break;
-- 
2.43.5