[PATCH v6 01/11] KVM: Add lockless memslot walk to KVM

[James Houghton]

Provide flexibility to the architecture to synchronize as optimally as
they can instead of always taking the MMU lock for writing.

Architectures that do their own locking must select
CONFIG_KVM_MMU_NOTIFIER_YOUNG_LOCKLESS.

The immediate application is to allow architectures to implement the
test/clear_young MMU notifiers more cheaply.

Suggested-by: Yu Zhao <yuzhao at google.com>
Signed-off-by: James Houghton <jthoughton at google.com>
---
 include/linux/kvm_host.h |  1 +
 virt/kvm/Kconfig         |  3 +++
 virt/kvm/kvm_main.c      | 26 +++++++++++++++++++-------
 3 files changed, 23 insertions(+), 7 deletions(-)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 689e8be873a7..8cd80f969cff 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -266,6 +266,7 @@ struct kvm_gfn_range {
 	gfn_t end;
 	union kvm_mmu_notifier_arg arg;
 	bool may_block;
+	bool lockless;
 };
 bool kvm_unmap_gfn_range(struct kvm *kvm, struct kvm_gfn_range *range);
 bool kvm_age_gfn(struct kvm *kvm, struct kvm_gfn_range *range);
diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig
index b14e14cdbfb9..632334861001 100644
--- a/virt/kvm/Kconfig
+++ b/virt/kvm/Kconfig
@@ -100,6 +100,9 @@ config KVM_GENERIC_MMU_NOTIFIER
        select MMU_NOTIFIER
        bool
 
+config KVM_MMU_NOTIFIER_YOUNG_LOCKLESS
+       bool
+
 config KVM_GENERIC_MEMORY_ATTRIBUTES
        depends on KVM_GENERIC_MMU_NOTIFIER
        bool
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index d0788d0a72cc..33f8997a5c29 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -555,6 +555,7 @@ struct kvm_mmu_notifier_range {
 	on_lock_fn_t on_lock;
 	bool flush_on_ret;
 	bool may_block;
+	bool lockless;
 };
 
 /*
@@ -609,6 +610,10 @@ static __always_inline kvm_mn_ret_t __kvm_handle_hva_range(struct kvm *kvm,
 			 IS_KVM_NULL_FN(range->handler)))
 		return r;
 
+	/* on_lock will never be called for lockless walks */
+	if (WARN_ON_ONCE(range->lockless && !IS_KVM_NULL_FN(range->on_lock)))
+		return r;
+
 	idx = srcu_read_lock(&kvm->srcu);
 
 	for (i = 0; i < kvm_arch_nr_memslot_as_ids(kvm); i++) {
@@ -640,15 +645,18 @@ static __always_inline kvm_mn_ret_t __kvm_handle_hva_range(struct kvm *kvm,
 			gfn_range.start = hva_to_gfn_memslot(hva_start, slot);
 			gfn_range.end = hva_to_gfn_memslot(hva_end + PAGE_SIZE - 1, slot);
 			gfn_range.slot = slot;
+			gfn_range.lockless = range->lockless;
 
 			if (!r.found_memslot) {
 				r.found_memslot = true;
-				KVM_MMU_LOCK(kvm);
-				if (!IS_KVM_NULL_FN(range->on_lock))
-					range->on_lock(kvm);
-
-				if (IS_KVM_NULL_FN(range->handler))
-					goto mmu_unlock;
+				if (!range->lockless) {
+					KVM_MMU_LOCK(kvm);
+					if (!IS_KVM_NULL_FN(range->on_lock))
+						range->on_lock(kvm);
+
+					if (IS_KVM_NULL_FN(range->handler))
+						goto mmu_unlock;
+				}
 			}
 			r.ret |= range->handler(kvm, &gfn_range);
 		}
@@ -658,7 +666,7 @@ static __always_inline kvm_mn_ret_t __kvm_handle_hva_range(struct kvm *kvm,
 		kvm_flush_remote_tlbs(kvm);
 
 mmu_unlock:
-	if (r.found_memslot)
+	if (r.found_memslot && !range->lockless)
 		KVM_MMU_UNLOCK(kvm);
 
 	srcu_read_unlock(&kvm->srcu, idx);
@@ -679,6 +687,8 @@ static __always_inline int kvm_handle_hva_range(struct mmu_notifier *mn,
 		.on_lock	= (void *)kvm_null_fn,
 		.flush_on_ret	= true,
 		.may_block	= false,
+		.lockless	=
+			IS_ENABLED(CONFIG_KVM_MMU_NOTIFIER_YOUNG_LOCKLESS),
 	};
 
 	return __kvm_handle_hva_range(kvm, &range).ret;
@@ -697,6 +707,8 @@ static __always_inline int kvm_handle_hva_range_no_flush(struct mmu_notifier *mn
 		.on_lock	= (void *)kvm_null_fn,
 		.flush_on_ret	= false,
 		.may_block	= false,
+		.lockless	=
+			IS_ENABLED(CONFIG_KVM_MMU_NOTIFIER_YOUNG_LOCKLESS),
 	};
 
 	return __kvm_handle_hva_range(kvm, &range).ret;
-- 
2.46.0.rc1.232.g9752f9e123-goog