linux-kernel - Re: [PATCH v2 2/6] KVM: x86/mmu: Properly account NX huge page workaround for nonpaging MMUs

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Date:   Thu, 28 Jul 2022 22:15:48 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Yosry Ahmed <yosryahmed@...gle.com>,
        Mingwei Zhang <mizhang@...gle.com>,
        Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH v2 2/6] KVM: x86/mmu: Properly account NX huge page
 workaround for nonpaging MMUs

On 7/23/22 03:23, Sean Christopherson wrote:
> +
> +	/*
> +	 * Note, enforcing the NX huge page mitigation for nonpaging MMUs
> +	 * (shadow paging, CR0.PG=0 in the guest) is completely unnecessary.
> +	 * The guest doesn't have any page tables to abuse and is guaranteed
> +	 * to switch to a different MMU when CR0.PG is toggled on (may not
> +	 * always be guaranteed when KVM is using TDP).  See also make_spte().
> +	 */
>   	const bool nx_huge_page_workaround_enabled;
>   
>   	/*
> diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
> index 7314d27d57a4..9f3e5af088a5 100644
> --- a/arch/x86/kvm/mmu/spte.c
> +++ b/arch/x86/kvm/mmu/spte.c
> @@ -147,6 +147,17 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
>   	if (!prefetch)
>   		spte |= spte_shadow_accessed_mask(spte);
>   
> +	/*
> +	 * For simplicity, enforce the NX huge page mitigation even if not
> +	 * strictly necessary.  KVM could ignore if the mitigation if paging is
> +	 * disabled in the guest, but KVM would then have to ensure a new MMU
> +	 * is loaded (or all shadow pages zapped) when CR0.PG is toggled on,
> +	 * and that's a net negative for performance when TDP is enabled.  KVM
> +	 * could ignore the mitigation if TDP is disabled and CR0.PG=0, as KVM
> +	 * will always switch to a new MMU if paging is enabled in the guest,
> +	 * but that adds complexity just to optimize a mode that is anything
> +	 * but performance critical.

Why even separate the two comments?  I think they both belong in 
make_spte().

Paolo