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

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Message-ID: <YvJ0EpvDhc00NTSx@google.com>
Date:   Tue, 9 Aug 2022 14:49:54 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.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 Tue, Aug 09, 2022, Paolo Bonzini wrote:
> 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.

If we can get away with it, I'd prefer to rely on the READ_ONCE() in
kvm_tdp_mmu_read_spte() and required ordering of:

	READ_ONCE() => PRESENT => spte_to_child_sp()