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.
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
arch/x86/kvm/mmu/mmu.c | 22 +++++++++++++++++++---
include/linux/kvm_host.h | 5 +++++
virt/kvm/kvm_main.c | 5 +++++
3 files changed, 29 insertions(+), 3 deletions(-)
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index cfff74685a25..d4ec9491d468 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_logpage_memslots 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_logpage_memslots)) {
+ 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,
}
}
+ 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));
+
r = kvm_arch_prepare_memory_region(kvm, old, new, change);
/* Free the bitmap on failure if it was allocated above. */
--
2.31.1
