linux-kernel - Re: [PATCH v2 3/6] KVM: x86/mmu: only zap EPT when guest MTRR changes

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Date:   Wed, 10 May 2023 13:39:25 +0800
From:   Chao Gao <chao.gao@...el.com>
To:     Yan Zhao <yan.y.zhao@...el.com>
CC:     <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <pbonzini@...hat.com>, <seanjc@...gle.com>
Subject: Re: [PATCH v2 3/6] KVM: x86/mmu: only zap EPT when guest MTRR changes

On Tue, May 09, 2023 at 09:51:43PM +0800, Yan Zhao wrote:
>Call new helper kvm_zap_gfn_for_memtype() to skip zap mmu if EPT is not
>enabled.
>
>When guest MTRR changes and it's desired to zap TDP entries to remove
>stale mappings, only do it when EPT is enabled, because only memory type
>of EPT leaf is affected by guest MTRR with noncoherent DMA present.
>
>Signed-off-by: Yan Zhao <yan.y.zhao@...el.com>
>---
> arch/x86/kvm/mtrr.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
>diff --git a/arch/x86/kvm/mtrr.c b/arch/x86/kvm/mtrr.c
>index 9fac1ec03463..62ebb9978156 100644
>--- a/arch/x86/kvm/mtrr.c
>+++ b/arch/x86/kvm/mtrr.c
>@@ -330,7 +330,7 @@ static void update_mtrr(struct kvm_vcpu *vcpu, u32 msr)
> 		var_mtrr_range(&mtrr_state->var_ranges[index], &start, &end);
> 	}
> 
>-	kvm_zap_gfn_range(vcpu->kvm, gpa_to_gfn(start), gpa_to_gfn(end));
>+	kvm_zap_gfn_for_memtype(vcpu->kvm, gpa_to_gfn(start), gpa_to_gfn(end));

I am wondering if the check of shadow_memtype_mask (now inside the
kvm_zap_gfn_for_memtype()) should be moved to the beginning of update_mtrr().
Because if EPT isn't enabled, computing @start/@end is useless and can be
skipped.

> }
> 
> static bool var_mtrr_range_is_valid(struct kvm_mtrr_range *range)
>-- 
>2.17.1
>