Re: [PATCH v3 8/8] KVM: x86/mmu: explicitly check nx_hugepage in disallowed_hugepage_adjust()

From: Sean Christopherson <seanjc@google.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	David Matlack <dmatlack@google.com>,
	Yan Zhao <yan.y.zhao@intel.com>,
	Mingwei Zhang <mizhang@google.com>,
	Ben Gardon <bgardon@google.com>
Subject: Re: [PATCH v3 8/8] KVM: x86/mmu: explicitly check nx_hugepage in disallowed_hugepage_adjust()
Date: Tue, 9 Aug 2022 14:49:54 +0000	[thread overview]
Message-ID: <YvJ0EpvDhc00NTSx@google.com> (raw)
In-Reply-To: <36634375-e7ee-e28e-20dd-9ab1ebdd8040@redhat.com>

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()