linux-kernel - Re: [PATCH v2 1/8] KVM: VMX: Open code writing vCPU's PAT in VMX's MSR handler

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Date:   Fri, 12 May 2023 10:18:35 +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 1/8] KVM: VMX: Open code writing vCPU's PAT in VMX's
 MSR handler

On Thu, 2023-05-11 at 16:33 -0700, Sean Christopherson wrote:
> From: Wenyao Hai <haiwenyao@...ontech.com>
> 
> Open code setting "vcpu->arch.pat" in vmx_set_msr() instead of bouncing
> through kvm_set_msr_common() to get to the same code in kvm_mtrr_set_msr().
> This aligns VMX with SVM, avoids hiding a very simple operation behind a
> relatively complicated function call (finding the PAT MSR case in
> kvm_set_msr_common() is non-trivial), and most importantly, makes it clear
> that not unwinding the VMCS updates if kvm_set_msr_common() isn't a bug
> (because kvm_set_msr_common() can never fail for PAT).
> 
> Opportunistically set vcpu->arch.pat before updating the VMCS info so that
> a future patch can move the common bits (back) into kvm_set_msr_common()
> without a functional change.
> 
> Note, MSR_IA32_CR_PAT is 0x277, and is very subtly handled by
> 
> 	case 0x200 ... MSR_IA32_MC0_CTL2 - 1:
> 
> in kvm_set_msr_common().
> 
> Cc: Kai Huang <kai.huang@...el.com>
> Signed-off-by: Wenyao Hai <haiwenyao@...ontech.com>
> [sean: massage changelog, hoist setting vcpu->arch.pat up]
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

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

> ---
>  arch/x86/kvm/vmx/vmx.c | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 44fb619803b8..33b8625d3541 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -2290,16 +2290,14 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  		if (!kvm_pat_valid(data))
>  			return 1;
>  
> +		vcpu->arch.pat = data;
> +
>  		if (is_guest_mode(vcpu) &&
>  		    get_vmcs12(vcpu)->vm_exit_controls & VM_EXIT_SAVE_IA32_PAT)
>  			get_vmcs12(vcpu)->guest_ia32_pat = data;
>  
> -		if (vmcs_config.vmentry_ctrl & VM_ENTRY_LOAD_IA32_PAT) {
> +		if (vmcs_config.vmentry_ctrl & VM_ENTRY_LOAD_IA32_PAT)
>  			vmcs_write64(GUEST_IA32_PAT, data);
> -			vcpu->arch.pat = data;
> -			break;
> -		}
> -		ret = kvm_set_msr_common(vcpu, msr_info);
>  		break;
>  	case MSR_IA32_MCG_EXT_CTL:
>  		if ((!msr_info->host_initiated &&
> -- 
> 2.40.1.606.ga4b1b128d6-goog
>