Re: [PATCH 01/23] KVM: MMU: pass uses_nx directly to reset_shadow_zero_bits_mask

David Matlack
2022-02-04T17:59:56Z

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.)

> ---
> arch/x86/kvm/mmu/mmu.c | 26 +++++++++++++-------------
> 1 file changed, 13 insertions(+), 13 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 296f8723f9ae..9424ae90f1ef 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4410,18 +4410,9 @@ static inline u64 reserved_hpa_bits(void)
> * follow the features in guest.
> */
> static void reset_shadow_zero_bits_mask(struct kvm_vcpu *vcpu,
> - struct kvm_mmu *context)
> + struct kvm_mmu *context,
> + bool uses_nx)
> {
> - /*
> - * 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.
> - */
> - bool uses_nx = is_efer_nx(context) || !tdp_enabled;
> -
> /* @amd adds a check on bit of SPTEs, which KVM shouldn't use anyways. */
> bool is_amd = true;
> /* KVM doesn't use 2-level page tables for the shadow MMU. */
> @@ -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);
> }
>
> 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);
> }
>
> 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, &regs, 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
>
>