From: Sean Christopherson <seanjc@google.com>
To: David Matlack <dmatlack@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
vkuznets@redhat.com
Subject: Re: [PATCH 01/23] KVM: MMU: pass uses_nx directly to reset_shadow_zero_bits_mask
Date: Thu, 10 Feb 2022 00:30:08 +0000 [thread overview]
Message-ID: <YgRckLixnxa7hLqB@google.com> (raw)
In-Reply-To: <Yf1pk1EEBXj0O0/p@google.com>
On Fri, Feb 04, 2022, David Matlack wrote:
> On Fri, Feb 04, 2022 at 06:56:56AM -0500, Paolo Bonzini wrote:
> > reset_shadow_zero_bits_mask has a very unintuitive way of deciding
> > whether the shadow pages will use the NX bit. The function is used in
> > two cases, shadow paging and shadow NPT; shadow paging has a use for
> > EFER.NX and needs to force it enabled, while shadow NPT only needs it
> > depending on L1's setting.
> >
> > The actual root problem here is that is_efer_nx, despite being part
> > of the "base" role, only matches the format of the shadow pages in the
> > NPT case. For now, just remove the ugly variable initialization and move
> > the call to reset_shadow_zero_bits_mask out of shadow_mmu_init_context.
> > The parameter can then be removed after the root problem in the role
> > is fixed.
> >
> > Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>
> Reviewed-by: David Matlack <dmatlack@google.com>
>
> (I agree this commit makes no functional change.)
There may not be a functional change, but it drops an optimization and contributes
to making future code/patches more fragile due to making it harder to understand
the relationship between shadow_mmu_init_context() and __kvm_mmu_new_pgd().
> > @@ -4829,8 +4820,6 @@ static void shadow_mmu_init_context(struct kvm_vcpu *vcpu, struct kvm_mmu *conte
> >
> > reset_guest_paging_metadata(vcpu, context);
> > context->shadow_root_level = new_role.base.level;
> > -
> > - reset_shadow_zero_bits_mask(vcpu, context);
This is guarded by:
if (new_role.as_u64 == context->mmu_role.as_u64)
return;
> > }
> >
> > static void kvm_init_shadow_mmu(struct kvm_vcpu *vcpu,
> > @@ -4841,6 +4830,16 @@ static void kvm_init_shadow_mmu(struct kvm_vcpu *vcpu,
> > kvm_calc_shadow_mmu_root_page_role(vcpu, regs, false);
> >
> > shadow_mmu_init_context(vcpu, context, regs, new_role);
> > +
> > + /*
> > + * KVM uses NX when TDP is disabled to handle a variety of scenarios,
> > + * notably for huge SPTEs if iTLB multi-hit mitigation is enabled and
> > + * to generate correct permissions for CR0.WP=0/CR4.SMEP=1/EFER.NX=0.
> > + * The iTLB multi-hit workaround can be toggled at any time, so assume
> > + * NX can be used by any non-nested shadow MMU to avoid having to reset
> > + * MMU contexts. Note, KVM forces EFER.NX=1 when TDP is disabled.
> > + */
> > + reset_shadow_zero_bits_mask(vcpu, context, true);
Whereas this will compute the mask even if the role doesn't change. I say that
matters later on because then this sequence:
shadow_mmu_init_context(vcpu, context, ®s, new_role);
reset_shadow_zero_bits_mask(vcpu, context, is_efer_nx(context));
__kvm_mmu_new_pgd(vcpu, nested_cr3, new_role.base);
becomes even more difficult to untangle.
And looking at where this series ends up, I don't understand the purpose of this
change. Patch 18 essentially reverts this patch, and I see nothing in between
that will break without the temporary change. That patch becomes:
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 02e6d256805d..f9c96de1189d 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -4408,7 +4408,7 @@ static void reset_shadow_zero_bits_mask(struct kvm_vcpu *vcpu,
* NX can be used by any non-nested shadow MMU to avoid having to reset
* MMU contexts. Note, KVM forces EFER.NX=1 when TDP is disabled.
*/
- bool uses_nx = is_efer_nx(context) || !tdp_enabled;
+ bool uses_nx = context->mmu_role.efer_nx;
/* @amd adds a check on bit of SPTEs, which KVM shouldn't use anyways. */
bool is_amd = true;
though it needs to update the comment as well.
> > }
> >
> > static union kvm_mmu_role
> > @@ -4872,6 +4871,7 @@ void kvm_init_shadow_npt_mmu(struct kvm_vcpu *vcpu, unsigned long cr0,
> > __kvm_mmu_new_pgd(vcpu, nested_cr3, new_role.base);
> >
> > shadow_mmu_init_context(vcpu, context, ®s, new_role);
> > + reset_shadow_zero_bits_mask(vcpu, context, is_efer_nx(context));
>
> Out of curiousity, how does KVM mitigate iTLB multi-hit when shadowing
> NPT and the guest has not enabled EFER.NX?
>
> > }
> > EXPORT_SYMBOL_GPL(kvm_init_shadow_npt_mmu);
> >
> > --
> > 2.31.1
> >
> >
next prev parent reply other threads:[~2022-02-10 1:32 UTC|newest]
Thread overview: 72+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-02-04 11:56 [PATCH 00/23] KVM: MMU: MMU role refactoring Paolo Bonzini
2022-02-04 11:56 ` [PATCH 01/23] KVM: MMU: pass uses_nx directly to reset_shadow_zero_bits_mask Paolo Bonzini
2022-02-04 17:59 ` David Matlack
2022-02-05 14:52 ` Paolo Bonzini
2022-02-07 16:09 ` Sean Christopherson
2022-02-07 21:50 ` David Matlack
2022-02-10 0:30 ` Sean Christopherson [this message]
2022-02-10 16:38 ` Paolo Bonzini
2022-02-04 11:56 ` [PATCH 02/23] KVM: MMU: nested EPT cannot be used in SMM Paolo Bonzini
2022-02-04 18:16 ` David Matlack
2022-02-09 22:43 ` Sean Christopherson
2022-02-04 11:56 ` [PATCH 03/23] KVM: MMU: remove valid from extended role Paolo Bonzini
2022-02-04 18:32 ` David Matlack
2022-02-05 14:50 ` Paolo Bonzini
2022-02-09 22:54 ` Sean Christopherson
2022-02-10 9:53 ` Paolo Bonzini
2022-02-04 11:56 ` [PATCH 04/23] KVM: MMU: constify uses of struct kvm_mmu_role_regs Paolo Bonzini
2022-02-04 18:41 ` David Matlack
2022-02-09 22:57 ` Sean Christopherson
2022-02-04 11:57 ` [PATCH 05/23] KVM: MMU: pull computation of kvm_mmu_role_regs to kvm_init_mmu Paolo Bonzini
2022-02-04 18:45 ` David Matlack
2022-02-04 11:57 ` [PATCH 06/23] KVM: MMU: load new PGD once nested two-dimensional paging is initialized Paolo Bonzini
2022-02-04 19:18 ` David Matlack
2022-02-07 13:50 ` Paolo Bonzini
2022-02-07 14:35 ` Paolo Bonzini
2022-02-09 12:34 ` Paolo Bonzini
2022-02-04 11:57 ` [PATCH 07/23] KVM: MMU: remove kvm_mmu_calc_root_page_role Paolo Bonzini
2022-02-04 19:32 ` David Matlack
2022-02-05 14:46 ` Paolo Bonzini
2022-02-10 0:47 ` Sean Christopherson
2022-02-10 9:52 ` Paolo Bonzini
2022-02-10 17:29 ` Sean Christopherson
2022-02-10 17:43 ` Paolo Bonzini
2022-02-04 11:57 ` [PATCH 08/23] KVM: MMU: rephrase unclear comment Paolo Bonzini
2022-02-04 19:38 ` David Matlack
2022-02-04 11:57 ` [PATCH 09/23] KVM: MMU: remove "bool base_only" arguments Paolo Bonzini
2022-02-04 19:41 ` David Matlack
2022-02-04 11:57 ` [PATCH 10/23] KVM: MMU: split cpu_role from mmu_role Paolo Bonzini
2022-02-04 21:57 ` David Matlack
2022-02-05 14:49 ` Paolo Bonzini
2022-02-07 21:38 ` David Matlack
2022-02-04 11:57 ` [PATCH 11/23] KVM: MMU: do not recompute root level from kvm_mmu_role_regs Paolo Bonzini
2022-02-07 22:10 ` David Matlack
2022-02-07 22:17 ` David Matlack
2022-02-04 11:57 ` [PATCH 12/23] KVM: MMU: remove ept_ad field Paolo Bonzini
2022-02-04 11:57 ` [PATCH 13/23] KVM: MMU: remove kvm_calc_shadow_root_page_role_common Paolo Bonzini
2022-02-07 22:25 ` David Matlack
2022-02-04 11:57 ` [PATCH 14/23] KVM: MMU: cleanup computation of MMU roles for two-dimensional paging Paolo Bonzini
2022-02-04 11:57 ` [PATCH 15/23] KVM: MMU: cleanup computation of MMU roles for shadow paging Paolo Bonzini
2022-02-04 11:57 ` [PATCH 16/23] KVM: MMU: remove extended bits from mmu_role Paolo Bonzini
2022-02-04 11:57 ` [PATCH 17/23] KVM: MMU: remove redundant bits from extended role Paolo Bonzini
2022-02-04 11:57 ` [PATCH 18/23] KVM: MMU: fetch shadow EFER.NX from MMU role Paolo Bonzini
2022-02-04 11:57 ` [PATCH 19/23] KVM: MMU: simplify and/or inline computation of shadow MMU roles Paolo Bonzini
2022-02-04 11:57 ` [PATCH 20/23] KVM: MMU: pull CPU role computation to kvm_init_mmu Paolo Bonzini
2022-02-07 22:42 ` David Matlack
2022-02-04 11:57 ` [PATCH 21/23] KVM: MMU: store shadow_root_level into mmu_role Paolo Bonzini
2022-02-07 23:00 ` David Matlack
2022-02-04 11:57 ` [PATCH 22/23] KVM: MMU: use cpu_role for root_level Paolo Bonzini
2022-02-07 23:01 ` David Matlack
2022-02-04 11:57 ` [PATCH 23/23] KVM: MMU: replace direct_map with mmu_role.direct Paolo Bonzini
2022-02-07 23:02 ` David Matlack
2022-02-07 23:08 ` [PATCH 00/23] KVM: MMU: MMU role refactoring David Matlack
2022-02-07 23:27 ` Sean Christopherson
2022-02-07 23:53 ` David Matlack
2022-02-10 1:11 ` Sean Christopherson
2022-02-10 11:58 ` Paolo Bonzini
2022-02-10 16:55 ` Sean Christopherson
2022-02-10 17:30 ` Paolo Bonzini
2022-02-10 19:28 ` Sean Christopherson
2022-02-09 22:31 ` Sean Christopherson
2022-02-10 9:54 ` Paolo Bonzini
2022-02-14 18:14 ` David Matlack
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