linux-kernel - Re: [PATCH v3 5/8] KVM: x86/mmu: Set disallowed_nx_huge

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Date:   Tue, 9 Aug 2022 14:49:23 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Yan Zhao <yan.y.zhao@...el.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        David Matlack <dmatlack@...gle.com>,
        Mingwei Zhang <mizhang@...gle.com>,
        Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH v3 5/8] KVM: x86/mmu: Set disallowed_nx_huge_page in TDP
 MMU before setting SPTE

On 8/9/22 05:26, Yan Zhao wrote:
> hi Sean,
> 
> I understand this smp_rmb() is intended to prevent the reading of
> p->nx_huge_page_disallowed from happening before it's set to true in
> kvm_tdp_mmu_map(). Is this understanding right?
> 
> If it's true, then do we also need the smp_rmb() for read of sp->gfn in
> handle_removed_pt()? (or maybe for other fields in sp in other places?)

No, in that case the barrier is provided by rcu_dereference().  In fact, 
I am not sure the barriers are needed in this patch either (but the 
comments are :)):

- the write barrier is certainly not needed because it is implicit in 
tdp_mmu_set_spte_atomic's cmpxchg64

- the read barrier _should_ also be provided by rcu_dereference(pt), but 
I'm not 100% sure about that. The reasoning is that you have

(1)	iter->old spte = READ_ONCE(*rcu_dereference(iter->sptep));
	...
(2)	tdp_ptep_t pt = spte_to_child_pt(old_spte, level);
(3)	struct kvm_mmu_page *sp = sptep_to_sp(rcu_dereference(pt));
	...
(4)	if (sp->nx_huge_page_disallowed) {

and (4) is definitely ordered after (1) thanks to the READ_ONCE hidden 
within (3) and the data dependency from old_spte to sp.

Paolo