Since gfn_to_memslot() is relatively expensive, it helps to skip it if it the memslot cannot possibly have dirty logging enabled. In order to do this, add to struct kvm a counter of the number of log-page memslots. While the correct value can only be read with slots_lock taken, the NX recovery thread is content with using an approximate value. Therefore, the counter is an atomic_t. Based on https://lore.kernel.org/kvm/20221027200316.2221027-2-dmatlack@xxxxxxxxxx/ by David Matlack. Supersedes: <20221117173109.3126912-1-pbonzini@xxxxxxxxxx> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx> --- v1->v2: actually works, using ideas from David's v1 arch/x86/kvm/mmu/mmu.c | 22 +++++++++++++++++++--- include/linux/kvm_host.h | 5 +++++ virt/kvm/kvm_main.c | 7 +++++++ 3 files changed, 31 insertions(+), 3 deletions(-) diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index cfff74685a25..4736d7849c60 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@ -6878,16 +6878,32 @@ static void kvm_recover_nx_huge_pages(struct kvm *kvm) WARN_ON_ONCE(!sp->nx_huge_page_disallowed); WARN_ON_ONCE(!sp->role.direct); - slot = gfn_to_memslot(kvm, sp->gfn); - WARN_ON_ONCE(!slot); - /* * Unaccount and do not attempt to recover any NX Huge Pages * that are being dirty tracked, as they would just be faulted * back in as 4KiB pages. The NX Huge Pages in this slot will be * recovered, along with all the other huge pages in the slot, * when dirty logging is disabled. + * + * Since gfn_to_memslot() is relatively expensive, it helps to + * skip it if it the test cannot possibly return true. On the + * other hand, if any memslot has logging enabled, chances are + * good that all of them do, in which case unaccount_nx_huge_page() + * is much cheaper than zapping the page. + * + * If a memslot update is in progress, reading an incorrect value + * of kvm->nr_memslots_dirty_logging is not a problem: if it is + * becoming zero, gfn_to_memslot() will be done unnecessarily; if + * it is becoming nonzero, the page will be zapped unnecessarily. + * Either way, this only affects efficiency in racy situations, + * and not correctness. */ + slot = NULL; + if (atomic_read(&kvm->nr_memslots_dirty_logging)) { + 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..6f0f389f5f9c 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_memslots_dirty_logging; /* 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..5d85f1a61793 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -1603,6 +1603,8 @@ static int kvm_prepare_memory_region(struct kvm *kvm, struct kvm_memory_slot *new, enum kvm_mr_change change) { + int old_flags = old ? old->flags : 0; + int new_flags = new ? new->flags : 0; int r; /* @@ -1627,6 +1629,11 @@ static int kvm_prepare_memory_region(struct kvm *kvm, } } + if ((old_flags ^ new_flags) & KVM_MEM_LOG_DIRTY_PAGES) { + int change = (new_flags & KVM_MEM_LOG_DIRTY_PAGES) ? 1 : -1; + atomic_set(&kvm->nr_memslots_dirty_logging, + atomic_read(&kvm->nr_memslots_dirty_logging) + change); + } r = kvm_arch_prepare_memory_region(kvm, old, new, change); /* Free the bitmap on failure if it was allocated above. */ -- 2.31.1
