linux-kernel - Re: [PATCH 01/23] KVM: MMU: pass uses_nx directly to reset_shadow_zero_bits

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Date:   Fri, 4 Feb 2022 17:59:47 +0000
From:   David Matlack <dmatlack@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        seanjc@...gle.com, vkuznets@...hat.com
Subject: Re: [PATCH 01/23] KVM: MMU: pass uses_nx directly to
 reset_shadow_zero_bits_mask

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@...hat.com>

Reviewed-by: David Matlack <dmatlack@...gle.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
> 
>