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

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Message-ID: <ZFtPKLvblLcxPvZQ@yzhao56-desk.sh.intel.com>
Date:   Wed, 10 May 2023 16:00:40 +0800
From:   Yan Zhao <yan.y.zhao@...el.com>
To:     Chao Gao <chao.gao@...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 Wed, May 10, 2023 at 01:39:25PM +0800, Chao Gao wrote:
> 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.

No, shadow_memtype_mask is not accessible in mtrr.c.
It's better to confine it in mmu related files, e.g. mmu/mmu.c,
mmu/spte.c

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