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

[Sean Christopherson <seanjc@google.com>]

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@google.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