From: Paolo Bonzini <pbonzini@redhat.com>
To: Yan Zhao <yan.y.zhao@intel.com>, Sean Christopherson <seanjc@google.com>
Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
David Matlack <dmatlack@google.com>,
Mingwei Zhang <mizhang@google.com>,
Ben Gardon <bgardon@google.com>
Subject: Re: [PATCH v3 5/8] KVM: x86/mmu: Set disallowed_nx_huge_page in TDP MMU before setting SPTE
Date: Tue, 9 Aug 2022 14:49:23 +0200 [thread overview]
Message-ID: <331dc774-c662-9475-1175-725cb2382bb2@redhat.com> (raw)
In-Reply-To: <YvHT0dA0BGgCQ8L+@yzhao56-desk.sh.intel.com>
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
next prev parent reply other threads:[~2022-08-09 12:49 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-08-05 23:05 [PATCH v3 0/8] KVM: x86: Apply NX mitigation more precisely Sean Christopherson
2022-08-05 23:05 ` [PATCH v3 1/8] KVM: x86/mmu: Bug the VM if KVM attempts to double count an NX huge page Sean Christopherson
2022-08-14 0:53 ` Mingwei Zhang
2022-08-05 23:05 ` [PATCH v3 2/8] KVM: x86/mmu: Tag disallowed NX huge pages even if they're not tracked Sean Christopherson
2022-08-14 0:53 ` Mingwei Zhang
2022-08-05 23:05 ` [PATCH v3 3/8] KVM: x86/mmu: Rename NX huge pages fields/functions for consistency Sean Christopherson
2022-08-14 1:12 ` Mingwei Zhang
2022-08-15 21:54 ` Sean Christopherson
2022-08-16 21:09 ` Mingwei Zhang
2022-08-17 16:13 ` Sean Christopherson
2022-08-18 22:13 ` Mingwei Zhang
2022-08-18 23:45 ` Sean Christopherson
2022-08-19 18:30 ` Mingwei Zhang
2022-08-20 1:04 ` Mingwei Zhang
2022-08-05 23:05 ` [PATCH v3 4/8] KVM: x86/mmu: Properly account NX huge page workaround for nonpaging MMUs Sean Christopherson
2022-08-16 21:25 ` Mingwei Zhang
2022-08-05 23:05 ` [PATCH v3 5/8] KVM: x86/mmu: Set disallowed_nx_huge_page in TDP MMU before setting SPTE Sean Christopherson
2022-08-09 3:26 ` Yan Zhao
2022-08-09 12:49 ` Paolo Bonzini [this message]
2022-08-09 14:44 ` Sean Christopherson
2022-08-09 14:48 ` Paolo Bonzini
2022-08-09 15:05 ` Sean Christopherson
2022-08-05 23:05 ` [PATCH v3 6/8] KVM: x86/mmu: Track the number of TDP MMU pages, but not the actual pages Sean Christopherson
2022-08-05 23:05 ` [PATCH v3 7/8] KVM: x86/mmu: Add helper to convert SPTE value to its shadow page Sean Christopherson
2022-08-05 23:05 ` [PATCH v3 8/8] KVM: x86/mmu: explicitly check nx_hugepage in disallowed_hugepage_adjust() Sean Christopherson
2022-08-09 12:57 ` Paolo Bonzini
2022-08-09 14:49 ` Sean Christopherson
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