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

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Message-ID: <ZFwzq/833QX3n9xc@yzhao56-desk.sh.intel.com>
Date:   Thu, 11 May 2023 08:15:39 +0800
From:   Yan Zhao <yan.y.zhao@...el.com>
To:     "Huang, Kai" <kai.huang@...el.com>
CC:     "Gao, Chao" <chao.gao@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Christopherson,, Sean" <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 06:54:51PM +0800, Huang, Kai wrote:
> On Wed, 2023-05-10 at 16:00 +0800, Yan Zhao wrote:
> > 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
> > 
> 
> No, I think it should be shadow_memtype_mask.
> 
> Conceptually, after commit 38bf9d7bf277 ("KVM: x86/mmu: Add shadow mask for
> effective host MTRR memtype"), I believe in update_mtrr() the check:
> 
> 	if (msr == MSR_IA32_CR_PAT || !tdp_enabled ||
>               !kvm_arch_has_noncoherent_dma(vcpu->kvm))
>                 return;
> 
> should be:
> 
> 	if (msr == MSR_IA32_CR_PAT || !shadow_memtype_mask ||
>               !kvm_arch_has_noncoherent_dma(vcpu->kvm))
>                 return;
> 
> Because the intention of 'shadow_memtype_mask' is to use it as a dedicated
> variable to represent hardware has capability to override the host MTRRs (which
> is the case for EPT), instead of relying on TDP being enabled.
> 
> That being said, to me it's more reasonable to have a separate patch to replace
> the '!tdp_enabled' check with '!shadow_memtype_mask' in update_mtrr(), perhaps
> with a Fixes tag for commit 38bf9d7bf277.
> 
> The kvm_zap_gfn_range() should be remain unchanged.
> 
> For the problem that shadow_memtype_mask isn't accessible in mtrr.c I think you
> can include "mmu/spte.h"?
> 

I agree shadow_memtype_mask is the right value to check but it's indeed
internal to kvm mmu.
Including "mmu/spte.h" will further include "mmu/mmu_internal.h"

What about introduce kvm_mmu_memtye_effective() instead as below?

diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
index a04577afbc71..173960b1827d 100644
--- a/arch/x86/kvm/mmu.h
+++ b/arch/x86/kvm/mmu.h
@@ -254,6 +254,8 @@ static inline bool kvm_shadow_root_allocated(struct kvm *kvm)
        return smp_load_acquire(&kvm->arch.shadow_root_allocated);
 }
 
+bool kvm_mmu_memtye_effective(struct kvm *kvm);
+
 #ifdef CONFIG_X86_64
 extern bool tdp_mmu_enabled;
 #else
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 2706754794d1..ff7752d158d7 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -6288,6 +6288,10 @@ void kvm_zap_gfn_for_memtype(struct kvm *kvm, gfn_t gfn_start, gfn_t gfn_end)
        kvm_zap_gfn_range(kvm, gpa_to_gfn(0), gpa_to_gfn(~0ULL));
 }
 
+bool kvm_mmu_memtye_effective(struct kvm *kvm)
+{
+       return shadow_memtype_mask;
+}
 /*
  * Invalidate (zap) SPTEs that cover GFNs from gfn_start and up to gfn_end
  * (not including it)
diff --git a/arch/x86/kvm/mtrr.c b/arch/x86/kvm/mtrr.c
index 5ce5bc0c4971..b6bd147d22a0 100644
--- a/arch/x86/kvm/mtrr.c
+++ b/arch/x86/kvm/mtrr.c
@@ -313,7 +313,7 @@ static void update_mtrr(struct kvm_vcpu *vcpu, u32 msr)
        int cnt;
        unsigned long long all;
 
-       if (msr == MSR_IA32_CR_PAT || !tdp_enabled ||
+       if (msr == MSR_IA32_CR_PAT || !kvm_mmu_memtye_effective(vcpu->kvm) ||
              !kvm_arch_has_noncoherent_dma(vcpu->kvm))
                return;