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[34.168.104.7]) by smtp.gmail.com with ESMTPSA id c65-20020a621c44000000b0056bcd7e1e04sm1760270pfc.124.2022.11.17.16.37.18 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 17 Nov 2022 16:37:18 -0800 (PST) Date: Fri, 18 Nov 2022 00:37:15 +0000 From: Sean Christopherson To: Paolo Bonzini Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, dmatlack@google.com Subject: Re: [PATCH] KVM: x86: avoid memslot check in NX hugepage recovery if it cannot be true Message-ID: References: <20221117173109.3126912-1-pbonzini@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20221117173109.3126912-1-pbonzini@redhat.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Nov 17, 2022, Paolo Bonzini wrote: > + if (atomic_read(&kvm->nr_logpage_memslots)) { Can we use something like nr_dirty_logged_memslots? logpage doesn't precisely capture the "dirty log" aspect, e.g. for a (very brief) second I though this was log(nr_memslot_pages). > + slot = gfn_to_memslot(kvm, sp->gfn); > + WARN_ON_ONCE(!slot); > + } > + > if (slot && kvm_slot_dirty_track_enabled(slot)) > unaccount_nx_huge_page(kvm, sp); > else if (is_tdp_mmu_page(sp)) > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h > index e6e66c5e56f2..b3c2b975e737 100644 > --- a/include/linux/kvm_host.h > +++ b/include/linux/kvm_host.h > @@ -722,6 +722,11 @@ struct kvm { > /* The current active memslot set for each address space */ > struct kvm_memslots __rcu *memslots[KVM_ADDRESS_SPACE_NUM]; > struct xarray vcpu_array; > + /* > + * Protected by slots_lock, but can be read outside if an > + * incorrect answer is acceptable. > + */ > + atomic_t nr_logpage_memslots; > > /* Used to wait for completion of MMU notifiers. */ > spinlock_t mn_invalidate_lock; > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c > index 43bbe4fde078..7670ebd29bcf 100644 > --- a/virt/kvm/kvm_main.c > +++ b/virt/kvm/kvm_main.c > @@ -1627,6 +1627,11 @@ static int kvm_prepare_memory_region(struct kvm *kvm, This needs to be done in the commit stage, e.g. if kvm_arch_prepare_memory_region() fails the count will be all kinds of wrong. Even better, since this seems to be x86-centric, put it in kvm_mmu_slot_apply_flags() under the if ((old_flags ^ new_flags) & KVM_MEM_LOG_DIRTY_PAGES) to avoid atomic operations if dirty logging isn't being toggled. That would also deal with the NULL pointer issues David pointed out. > } > } > > + atomic_set(&kvm->nr_logpage_memslots, > + atomic_read(&kvm->nr_logpage_memslots) > + + !!(new->flags & KVM_MEM_LOG_DIRTY_PAGES) > + - !!(old->flags & KVM_MEM_LOG_DIRTY_PAGES)); I belive this can be: atomic_add(+ !!(new_flags & KVM_MEM_LOG_DIRTY_PAGES) - !!(old_flags & KVM_MEM_LOG_DIRTY_PAGES), ...); or less weirdly... if ((old_flags ^ new_flags) & KVM_MEM_LOG_DIRTY_PAGES) { ... if (new_flags & KVM_MEM_LOG_DIRTY_PAGES) atomic_inc(...); else atomic_dec(...); }