linux-kernel - Re: [PATCH v2 8/8] KVM: x86: Move common handling of PAT MSR writes to kvm_set_msr

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Date:   Fri, 12 May 2023 10:48:20 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>
CC:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "guoke@...ontech.com" <guoke@...ontech.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "haiwenyao@...ontech.com" <haiwenyao@...ontech.com>
Subject: Re: [PATCH v2 8/8] KVM: x86: Move common handling of PAT MSR writes
 to kvm_set_msr_common()

On Thu, 2023-05-11 at 16:33 -0700, Sean Christopherson wrote:
> Move the common check-and-set handling of PAT MSR writes out of vendor
> code and into kvm_set_msr_common().  This aligns writes with reads, which
> are already handled in common code, i.e. makes the handling of reads and
> writes symmetrical in common code.
> 
> Alternatively, the common handling in kvm_get_msr_common() could be moved
> to vendor code, but duplicating code is generally undesirable (even though
> the duplicatated code is trivial in this case), and guest writes to PAT
> should be rare, i.e. the overhead of the extra function call is a
> non-issue in practice.
> 
> Suggested-by: Kai Huang <kai.huang@...el.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: Kai Huang <kai.huang@...el.com>

> ---
>  arch/x86/kvm/svm/svm.c | 7 ++++---
>  arch/x86/kvm/vmx/vmx.c | 7 +++----
>  arch/x86/kvm/x86.c     | 6 ------
>  3 files changed, 7 insertions(+), 13 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index db237ccdc957..61d329760f6c 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -2935,9 +2935,10 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
>  
>  		break;
>  	case MSR_IA32_CR_PAT:
> -		if (!kvm_pat_valid(data))
> -			return 1;
> -		vcpu->arch.pat = data;
> +		ret = kvm_set_msr_common(vcpu, msr);
> +		if (ret)
> +			break;
> +
>  		svm->vmcb01.ptr->save.g_pat = data;
>  		if (is_guest_mode(vcpu))
>  			nested_vmcb02_compute_g_pat(svm);
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 33b8625d3541..2d9d155691a7 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -2287,10 +2287,9 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  			return 1;
>  		goto find_uret_msr;
>  	case MSR_IA32_CR_PAT:
> -		if (!kvm_pat_valid(data))
> -			return 1;
> -
> -		vcpu->arch.pat = data;
> +		ret = kvm_set_msr_common(vcpu, msr_info);
> +		if (ret)
> +			break;
>  
>  		if (is_guest_mode(vcpu) &&
>  		    get_vmcs12(vcpu)->vm_exit_controls & VM_EXIT_SAVE_IA32_PAT)
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index d71cf924cd8f..3759737c0873 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3701,12 +3701,6 @@ 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);
> -
>  		if (!kvm_pat_valid(data))
>  			return 1;
>  
> -- 
> 2.40.1.606.ga4b1b128d6-goog
>