KVM: Take mmu_lock when handling MMU notifier iff the hva hits a memslot

Defer acquiring mmu_lock in the MMU notifier paths until a "hit" has been
detected in the memslots, i.e. don't take the lock for notifications that
don't affect the guest.

For small VMs, spurious locking is a minor annoyance.  And for "volatile"
setups where the majority of notifications _are_ relevant, this barely
qualifies as an optimization.

But, for large VMs (hundreds of threads) with static setups, e.g. no
page migration, no swapping, etc..., the vast majority of MMU notifier
callbacks will be unrelated to the guest, e.g. will often be in response
to the userspace VMM adjusting its own virtual address space.  In such
large VMs, acquiring mmu_lock can be painful as it blocks vCPUs from
handling page faults.  In some scenarios it can even be "fatal" in the
sense that it causes unacceptable brownouts, e.g. when rebuilding huge
pages after live migration, a significant percentage of vCPUs will be
attempting to handle page faults....

virt/kvm/kvm_main.c

@@ -482,10 +482,10 @@ static void kvm_null_fn(void)
 static __always_inline int __kvm_handle_hva_range(struct kvm *kvm,
 						  const struct kvm_hva_range *range)
 {
+	bool ret = false, locked = false;
 	struct kvm_gfn_range gfn_range;
 	struct kvm_memory_slot *slot;
 	struct kvm_memslots *slots;
-	bool ret = false;
 	int i, idx;
 
 	/* A null handler is allowed if and only if on_lock() is provided. */
@@ -493,11 +493,13 @@ static __always_inline int __kvm_handle_hva_range(struct kvm *kvm,
 			 IS_KVM_NULL_FN(range->handler)))
 		return 0;
 
-	KVM_MMU_LOCK(kvm);
-
 	idx = srcu_read_lock(&kvm->srcu);
 
+	/* The on_lock() path does not yet support lock elision. */
 	if (!IS_KVM_NULL_FN(range->on_lock)) {
+		locked = true;
+		KVM_MMU_LOCK(kvm);
+
 		range->on_lock(kvm, range->start, range->end);
 
 		if (IS_KVM_NULL_FN(range->handler))
@@ -532,6 +534,10 @@ static __always_inline int __kvm_handle_hva_range(struct kvm *kvm,
 			gfn_range.end = hva_to_gfn_memslot(hva_end + PAGE_SIZE - 1, slot);
 			gfn_range.slot = slot;
 
+			if (!locked) {
+				locked = true;
+				KVM_MMU_LOCK(kvm);
+			}
 			ret |= range->handler(kvm, &gfn_range);
 		}
 	}
@@ -540,7 +546,8 @@ static __always_inline int __kvm_handle_hva_range(struct kvm *kvm,
 		kvm_flush_remote_tlbs(kvm);
 
 out_unlock:
-	KVM_MMU_UNLOCK(kvm);
+	if (locked)
+		KVM_MMU_UNLOCK(kvm);
 
 	srcu_read_unlock(&kvm->srcu, idx);