diff mbox series

[v8,02/11] KVM: Add lockless memslot walk to KVM

Message ID 20241105184333.2305744-3-jthoughton@google.com (mailing list archive)
State New
Headers show
Series KVM: x86/mmu: Age sptes locklessly | expand

Commit Message

James Houghton Nov. 5, 2024, 6:43 p.m. UTC 
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@google.com>
Signed-off-by: James Houghton <jthoughton@google.com>
Reviewed-by: David Matlack <dmatlack@google.com>
---
 include/linux/kvm_host.h |  1 +
 virt/kvm/Kconfig         |  2 ++
 virt/kvm/kvm_main.c      | 28 +++++++++++++++++++++-------
 3 files changed, 24 insertions(+), 7 deletions(-)
diff mbox series

Patch

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 18a1672ffcbf..ab0318dbb8bd 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -260,6 +260,7 @@  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 54e959e7d68f..b50e4e629ac9 100644
--- a/virt/kvm/Kconfig
+++ b/virt/kvm/Kconfig
@@ -102,6 +102,8 @@  config KVM_GENERIC_MMU_NOTIFIER
 
 config KVM_ELIDE_TLB_FLUSH_IF_YOUNG
        depends on KVM_GENERIC_MMU_NOTIFIER
+
+config KVM_MMU_NOTIFIER_YOUNG_LOCKLESS
        bool
 
 config KVM_GENERIC_MEMORY_ATTRIBUTES
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 8b234a9acdb3..218edf037917 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -517,6 +517,7 @@  struct kvm_mmu_notifier_range {
 	on_lock_fn_t on_lock;
 	bool flush_on_ret;
 	bool may_block;
+	bool lockless;
 };
 
 /*
@@ -571,6 +572,10 @@  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++) {
@@ -602,15 +607,18 @@  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);
 		}
@@ -620,7 +628,7 @@  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);
@@ -797,6 +805,8 @@  static int kvm_mmu_notifier_clear_flush_young(struct mmu_notifier *mn,
 		.flush_on_ret	=
 			!IS_ENABLED(CONFIG_KVM_ELIDE_TLB_FLUSH_IF_YOUNG),
 		.may_block	= false,
+		.lockless	=
+			IS_ENABLED(CONFIG_KVM_MMU_NOTIFIER_YOUNG_LOCKLESS),
 	};
 
 	trace_kvm_age_hva(start, end);
@@ -817,6 +827,8 @@  static int kvm_mmu_notifier_clear_young(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),
 	};
 
 	trace_kvm_age_hva(start, end);
@@ -849,6 +861,8 @@  static int kvm_mmu_notifier_test_young(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),
 	};
 
 	trace_kvm_test_age_hva(address);