[PATCH v2] KVM: MMU: Don't use RCU for lockless shadow walking

[Avi Kivity <avi@redhat.com>]

Using RCU for lockless shadow walking can increase the amount of memory
in use by the system, since RCU grace periods are unpredictable.  We also
have an unconditional write to a shared variable (reader_counter), which
isn't good for scaling.

Replace that with a scheme similar to x86's get_user_pages_fast(): disable
interrupts during lockless shadow walk to force the freer
(kvm_mmu_commit_zap_page()) to wait for the TLB flush IPI to find the
processor with interrupts enabled.

We also add a new vcpu->mode, READING_SHADOW_PAGE_TABLES, to prevent
kvm_flush_remote_tlbs() from avoiding the IPI.

Signed-off-by: Avi Kivity <avi@redhat.com>
---
 arch/x86/include/asm/kvm_host.h |    4 ---
 arch/x86/kvm/mmu.c              |   72 +++++++++++++++------------------------
 include/linux/kvm_host.h        |    3 +-
 3 files changed, 30 insertions(+), 49 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index f624ca7..67e66e6 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -237,8 +237,6 @@ struct kvm_mmu_page {
 #endif
 
 	int write_flooding_count;
-
-	struct rcu_head rcu;
 };
 
 struct kvm_pio_request {
@@ -536,8 +534,6 @@ struct kvm_arch {
 	u64 hv_guest_os_id;
 	u64 hv_hypercall;
 
-	atomic_t reader_counter;
-
 	#ifdef CONFIG_KVM_MMU_AUDIT
 	int audit_point;
 	#endif
diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 07424cf..ef88034 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -551,19 +551,28 @@ static u64 mmu_spte_get_lockless(u64 *sptep)
 
 static void walk_shadow_page_lockless_begin(struct kvm_vcpu *vcpu)
 {
-	rcu_read_lock();
-	atomic_inc(&vcpu->kvm->arch.reader_counter);
-
-	/* Increase the counter before walking shadow page table */
-	smp_mb__after_atomic_inc();
+	/*
+	 * Prevent page table teardown by making any free-er wait during
+	 * kvm_flush_remote_tlbs() IPI to all active vcpus.
+	 */
+	local_irq_disable();
+	vcpu->mode = READING_SHADOW_PAGE_TABLES;
+	/*
+	 * wmb: advertise vcpu->mode change
+	 * rmb: make sure we see updated sptes
+	 */
+	smp_mb();
 }
 
 static void walk_shadow_page_lockless_end(struct kvm_vcpu *vcpu)
 {
-	/* Decrease the counter after walking shadow page table finished */
-	smp_mb__before_atomic_dec();
-	atomic_dec(&vcpu->kvm->arch.reader_counter);
-	rcu_read_unlock();
+	/*
+	 * Make our reads and writes to shadow page tables globally visible
+	 * before leaving READING_SHADOW_PAGE_TABLES mode.
+	 */
+	smp_mb();
+	vcpu->mode = OUTSIDE_GUEST_MODE;
+	local_irq_enable();
 }
 
 static int mmu_topup_memory_cache(struct kvm_mmu_memory_cache *cache,
@@ -1989,30 +1998,6 @@ static int kvm_mmu_prepare_zap_page(struct kvm *kvm, struct kvm_mmu_page *sp,
 	return ret;
 }
 
-static void kvm_mmu_isolate_pages(struct list_head *invalid_list)
-{
-	struct kvm_mmu_page *sp;
-
-	list_for_each_entry(sp, invalid_list, link)
-		kvm_mmu_isolate_page(sp);
-}
-
-static void free_pages_rcu(struct rcu_head *head)
-{
-	struct kvm_mmu_page *next, *sp;
-
-	sp = container_of(head, struct kvm_mmu_page, rcu);
-	while (sp) {
-		if (!list_empty(&sp->link))
-			next = list_first_entry(&sp->link,
-				      struct kvm_mmu_page, link);
-		else
-			next = NULL;
-		kvm_mmu_free_page(sp);
-		sp = next;
-	}
-}
-
 static void kvm_mmu_commit_zap_page(struct kvm *kvm,
 				    struct list_head *invalid_list)
 {
@@ -2021,17 +2006,17 @@ static void kvm_mmu_commit_zap_page(struct kvm *kvm,
 	if (list_empty(invalid_list))
 		return;
 
-	kvm_flush_remote_tlbs(kvm);
-
-	if (atomic_read(&kvm->arch.reader_counter)) {
-		kvm_mmu_isolate_pages(invalid_list);
-		sp = list_first_entry(invalid_list, struct kvm_mmu_page, link);
-		list_del_init(invalid_list);
+	/*
+	 * wmb: make sure everyone sees our modifications to the page tables
+	 * rmb: make sure we see changes to vcpu->mode
+	 */
+	smp_mb();
 
-		trace_kvm_mmu_delay_free_pages(sp);
-		call_rcu(&sp->rcu, free_pages_rcu);
-		return;
-	}
+	/*
+	 * Wait for all vcpus to exit guest mode and/or lockless shadow
+	 * page table walks.
+	 */
+	kvm_flush_remote_tlbs(kvm);
 
 	do {
 		sp = list_first_entry(invalid_list, struct kvm_mmu_page, link);
@@ -2039,7 +2024,6 @@ static void kvm_mmu_commit_zap_page(struct kvm *kvm,
 		kvm_mmu_isolate_page(sp);
 		kvm_mmu_free_page(sp);
 	} while (!list_empty(invalid_list));
-
 }
 
 /*
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 186ffab..d1f1adf 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -128,7 +128,8 @@ int kvm_async_pf_wakeup_all(struct kvm_vcpu *vcpu);
 enum {
 	OUTSIDE_GUEST_MODE,
 	IN_GUEST_MODE,
-	EXITING_GUEST_MODE
+	EXITING_GUEST_MODE,
+	READING_SHADOW_PAGE_TABLES,
 };
 
 /*
-- 
1.7.10