Messages in this thread Patch in this message | | | Date | Thu, 10 Feb 2022 00:30:08 +0000 | From | Sean Christopherson <> | Subject | Re: [PATCH 01/23] KVM: MMU: pass uses_nx directly to reset_shadow_zero_bits_mask |
| |
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 > > > >
| |