linux-kernel - Re: [PATCH 16/54] KVM: x86/mmu: Drop smep_andnot

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Date:   Wed, 23 Jun 2021 19:11:57 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        Maxim Levitsky <mlevitsk@...hat.com>
Subject: Re: [PATCH 16/54] KVM: x86/mmu: Drop smep_andnot_wp check from "uses
 NX" for shadow MMUs

On 22/06/21 19:57, Sean Christopherson wrote:
> Drop the smep_andnot_wp role check from the "uses NX" calculation now
> that all non-nested shadow MMUs treat NX as used via the !TDP check.
> 
> The shadow MMU for nested NPT, which shares the helper, does not need to
> deal with SMEP (or WP) as NPT walks are always "user" accesses and WP is
> explicitly noted as being ignored:
> 
>    Table walks for guest page tables are always treated as user writes at
>    the nested page table level.
> 
>    A table walk for the guest page itself is always treated as a user
>    access at the nested page table level
> 
>    The host hCR0.WP bit is ignored under nested paging.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>   arch/x86/kvm/mmu/mmu.c | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 96c16a6e0044..ca7680d1ea24 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4223,8 +4223,7 @@ reset_shadow_zero_bits_mask(struct kvm_vcpu *vcpu, struct kvm_mmu *context)
>   	 * 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 = context->nx || !tdp_enabled ||
> -		context->mmu_role.base.smep_andnot_wp;
> +	bool uses_nx = context->nx || !tdp_enabled;
>   	struct rsvd_bits_validate *shadow_zero_check;
>   	int i;
>   
> 

Good idea, but why not squash it into patch 2?

Paolo