linux-kernel - [PATCH 4/5] KVM: x86: WARN if writes to PAT MSR are handled by common KVM code

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Date:   Wed,  3 May 2023 11:28:51 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Wenyao Hai <haiwenyao@...ontech.com>,
        Ke Guo <guoke@...ontech.com>
Subject: [PATCH 4/5] KVM: x86: WARN if writes to PAT MSR are handled by common
 KVM code

WARN and continue if a write to the PAT MSR reaches kvm_set_msr_common()
now that both VMX and SVM handle PAT writes entirely on their own.  Keep
the case statement with a WARN instead of dropping it entirely to document
why KVM's handling of reads and writes isn't symmetrical (reads are still
handled by kvm_get_msr_common().

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/x86.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 8b356c9d8a81..c36256d00250 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3701,6 +3701,12 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		}
 		break;
 	case MSR_IA32_CR_PAT:
+		/*
+		 * Writes to PAT should be handled by vendor code as both SVM
+		 * and VMX track the guest's PAT in the VMCB/VMCS.
+		 */
+		WARN_ON_ONCE(1);
+		fallthrough;
 	case MTRRphysBase_MSR(0) ... MSR_MTRRfix4K_F8000:
 	case MSR_MTRRdefType:
 		return kvm_mtrr_set_msr(vcpu, msr, data);
-- 
2.40.1.495.gc816e09b53d-goog