linux-kernel - Re: [PATCH 09/15] KVM: SVM: Remove the unneeded code to mark available for CR3

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Date:   Thu, 18 Nov 2021 16:17:56 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     Lai Jiangshan <laijs@...ux.alibaba.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 09/15] KVM: SVM: Remove the unneeded code to mark
 available for CR3

On 11/8/21 13:44, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@...ux.alibaba.com>
> 
> VCPU_EXREG_CR3 is never cleared from vcpu->arch.regs_avail in SVM so
> marking available for CR3 is mere an NOP, just remove it.
> 
> And it is not required to mark it dirty since VCPU_EXREG_CR3 is neither
> never cleared from vcpu->arch.regs_dirty and SVM doesn't use the dirty
> information of VCPU_EXREG_CR3.
> 
> Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>
> ---
>   arch/x86/kvm/svm/nested.c | 1 -
>   1 file changed, 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index 13a58722e097..2d88ff584d61 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -444,7 +444,6 @@ static int nested_svm_load_cr3(struct kvm_vcpu *vcpu, unsigned long cr3,
>   		kvm_mmu_new_pgd(vcpu, cr3);
>   
>   	vcpu->arch.cr3 = cr3;
> -	kvm_register_mark_available(vcpu, VCPU_EXREG_CR3);
>   
>   	/* Re-initialize the MMU, e.g. to pick up CR4 MMU role changes. */
>   	kvm_init_mmu(vcpu);
> 

These two patches can be merged, I think, with a commit message like

     VCPU_EXREG_CR3 is never cleared from vcpu->arch.regs_avail or
     vcpu->arch.regs_dirty in SVM; therefore, marking CR3 as available is
     merely a NOP, and testing it will likewise always succeed.

Paolo