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

[Huang, Kai]

On Wed, 2023-05-03 at 11:28 -0700, Sean Christopherson wrote:
> 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().

Why not just merge this patch with the next one? 

> 
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  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);