linux-kernel - Re: [PATCH v3 8/8] KVM: x86/mmu: explicitly check nx_hugepage in disallowed_hugepage

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Date:   Tue, 9 Aug 2022 14:57:31 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        David Matlack <dmatlack@...gle.com>,
        Yan Zhao <yan.y.zhao@...el.com>,
        Mingwei Zhang <mizhang@...gle.com>,
        Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH v3 8/8] KVM: x86/mmu: explicitly check nx_hugepage in
 disallowed_hugepage_adjust()

On 8/6/22 01:05, Sean Christopherson wrote:
>   	    !is_large_pte(spte)) {
> +		u64 page_mask;
> +
> +		/*
> +		 * Ensure nx_huge_page_disallowed is read after checking for a
> +		 * present shadow page.  A different vCPU may be concurrently
> +		 * installing the shadow page if mmu_lock is held for read.
> +		 * Pairs with the smp_wmb() in kvm_tdp_mmu_map().
> +		 */
> +		smp_rmb();
> +
> +		if (!spte_to_child_sp(spte)->nx_huge_page_disallowed)
> +			return;
> +

I wonder if the barrier shouldn't be simply in to_shadow_page(), i.e. 
always assume in the TDP MMU code that sp->xyz is read after the SPTE 
that points to that struct kvm_mmu_page.

Paolo