linux-kernel - Re: [PATCH v3 03/11] KVM: x86/mmu: Use KVM honors guest MTRRs helper when CR0.CD toggles

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Message-ID: <ZJytJB+mwdU6v/XJ@google.com>
Date:   Wed, 28 Jun 2023 14:59:00 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Yan Zhao <yan.y.zhao@...el.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        pbonzini@...hat.com, chao.gao@...el.com, kai.huang@...el.com,
        robert.hoo.linux@...il.com
Subject: Re: [PATCH v3 03/11] KVM: x86/mmu: Use KVM honors guest MTRRs helper
 when CR0.CD toggles

On Fri, Jun 16, 2023, Yan Zhao wrote:
> Call helper to check if guest MTRRs are honored by KVM MMU before zapping,

Nit, state the effect, not what the code literally does.  The important part is
that the end result is that KVM will zap if and only if guest MTRRs are being
honored, e.g.

  Zap SPTEs when CR0.CD is toggled if and only if KVM's MMU is honoring
  guest MTRRs, which is the only time that KVM incorporates the guest's
  CR0.CD into the final memtype.

> as values of guest CR0.CD will only affect memory types of KVM TDP when
> guest MTRRs are honored.
> 
> Suggested-by: Chao Gao <chao.gao@...el.com>
> Signed-off-by: Yan Zhao <yan.y.zhao@...el.com>
> ---
>  arch/x86/kvm/x86.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 9e7186864542..6693daeb5686 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -942,7 +942,7 @@ void kvm_post_set_cr0(struct kvm_vcpu *vcpu, unsigned long old_cr0, unsigned lon
>  		kvm_mmu_reset_context(vcpu);
>  
>  	if (((cr0 ^ old_cr0) & X86_CR0_CD) &&
> -	    kvm_arch_has_noncoherent_dma(vcpu->kvm) &&
> +	    kvm_mmu_honors_guest_mtrrs(vcpu->kvm) &&
>  	    !kvm_check_has_quirk(vcpu->kvm, KVM_X86_QUIRK_CD_NW_CLEARED))
>  		kvm_zap_gfn_range(vcpu->kvm, 0, ~0ULL);
>  }
> -- 
> 2.17.1
>