Messages in this thread | | | Date | Tue, 9 Aug 2022 16:48:11 +0200 | Subject | Re: [PATCH v3 5/8] KVM: x86/mmu: Set disallowed_nx_huge_page in TDP MMU before setting SPTE | From | Paolo Bonzini <> |
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On 8/9/22 16:44, Sean Christopherson wrote: > On Tue, Aug 09, 2022, Paolo Bonzini wrote: >> 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 :)): > > Yeah, I'm 99% certain the barriers aren't strictly required, but I didn't love the > idea of depending on other implementation details for the barriers. Of course I > completely overlooked the fact that all other sp fields would need the same > barriers... > >> - 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. > > Yes, I think that's correct. Callers must verify the SPTE is present before getting > the associated child shadow page. KVM does have instances where a shadow page is > retrieved from the SPTE _pointer_, but that's the parent shadow page, i.e. isn't > guarded by the SPTE being present. > > struct kvm_mmu_page *sp = sptep_to_sp(rcu_dereference(iter->sptep)); > > Something like this is as a separate patch?
Would you resubmit without the memory barriers then?
> diff --git a/arch/x86/kvm/mmu/tdp_iter.h b/arch/x86/kvm/mmu/tdp_iter.h > index f0af385c56e0..9d982ccf4567 100644 > --- a/arch/x86/kvm/mmu/tdp_iter.h > +++ b/arch/x86/kvm/mmu/tdp_iter.h > @@ -13,6 +13,12 @@ > * to be zapped while holding mmu_lock for read, and to allow TLB flushes to be > * batched without having to collect the list of zapped SPs. Flows that can > * remove SPs must service pending TLB flushes prior to dropping RCU protection. > + * > + * The READ_ONCE() ensures that, if the SPTE points at a child shadow page, all > + * fields in struct kvm_mmu_page will be read after the caller observes the > + * present SPTE (KVM must check that the SPTE is present before following the > + * SPTE's pfn to its associated shadow page). Pairs with the implicit memory
I guess you mean both the shadow page table itself and the struct kvm_mmu_page? Or do you think to_shadow_page() should have a smp_rmb()?
Paolo
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