KVM: x86/mmu: Lazily allocate memslot rmaps

If the TDP MMU is in use, wait to allocate the rmaps until the shadow
MMU is actually used. (i.e. a nested VM is launched.) This saves memory
equal to 0.2% of guest memory in cases where the TDP MMU is used and
there are no nested guests involved.

Signed-off-by: Ben Gardon <bgardon@google.com>
Message-Id: <20210518173414.450044-8-bgardon@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -1869,4 +1869,6 @@ static inline int kvm_cpu_get_apicid(int mps_cpu)
 
 int kvm_cpu_dirty_log_size(void);
 
+int alloc_all_memslots_rmaps(struct kvm *kvm);
+
 #endif /* _ASM_X86_KVM_HOST_H */

arch/x86/kvm/mmu.h

@@ -234,7 +234,12 @@ void kvm_mmu_pre_destroy_vm(struct kvm *kvm);
 
 static inline bool kvm_memslots_have_rmaps(struct kvm *kvm)
 {
-	return kvm->arch.memslots_have_rmaps;
+	/*
+	 * Read memslot_have_rmaps before rmap pointers.  Hence, threads reading
+	 * memslots_have_rmaps in any lock context are guaranteed to see the
+	 * pointers.  Pairs with smp_store_release in alloc_all_memslots_rmaps.
+	 */
+	return smp_load_acquire(&kvm->arch.memslots_have_rmaps);
 }
 
 #endif

arch/x86/kvm/mmu/mmu.c

@@ -3312,6 +3312,10 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
 		}
 	}
 
+	r = alloc_all_memslots_rmaps(vcpu->kvm);
+	if (r)
+		return r;
+
 	write_lock(&vcpu->kvm->mmu_lock);
 	r = make_mmu_pages_available(vcpu);
 	if (r < 0)
@@ -5523,9 +5527,13 @@ void kvm_mmu_init_vm(struct kvm *kvm)
 {
 	struct kvm_page_track_notifier_node *node = &kvm->arch.mmu_sp_tracker;
 
-	kvm_mmu_init_tdp_mmu(kvm);
-
-	kvm->arch.memslots_have_rmaps = true;
+	if (!kvm_mmu_init_tdp_mmu(kvm))
+		/*
+		 * No smp_load/store wrappers needed here as we are in
+		 * VM init and there cannot be any memslots / other threads
+		 * accessing this struct kvm yet.
+		 */
+		kvm->arch.memslots_have_rmaps = true;
 
 	node->track_write = kvm_mmu_pte_write;
 	node->track_flush_slot = kvm_mmu_invalidate_zap_pages_in_memslot;

arch/x86/kvm/mmu/tdp_mmu.c

@@ -14,17 +14,19 @@ static bool __read_mostly tdp_mmu_enabled = false;
 module_param_named(tdp_mmu, tdp_mmu_enabled, bool, 0644);
 
 /* Initializes the TDP MMU for the VM, if enabled. */
-void kvm_mmu_init_tdp_mmu(struct kvm *kvm)
+bool kvm_mmu_init_tdp_mmu(struct kvm *kvm)
 {
 	if (!tdp_enabled || !READ_ONCE(tdp_mmu_enabled))
-		return;
+		return false;
 
 	/* This should not be changed for the lifetime of the VM. */
 	kvm->arch.tdp_mmu_enabled = true;
 
 	INIT_LIST_HEAD(&kvm->arch.tdp_mmu_roots);
 	spin_lock_init(&kvm->arch.tdp_mmu_pages_lock);
 	INIT_LIST_HEAD(&kvm->arch.tdp_mmu_pages);
+
+	return true;
 }
 
 static __always_inline void kvm_lockdep_assert_mmu_lock_held(struct kvm *kvm,

arch/x86/kvm/mmu/tdp_mmu.h

@@ -81,12 +81,12 @@ int kvm_tdp_mmu_get_walk(struct kvm_vcpu *vcpu, u64 addr, u64 *sptes,
 			 int *root_level);
 
 #ifdef CONFIG_X86_64
-void kvm_mmu_init_tdp_mmu(struct kvm *kvm);
+bool kvm_mmu_init_tdp_mmu(struct kvm *kvm);
 void kvm_mmu_uninit_tdp_mmu(struct kvm *kvm);
 static inline bool is_tdp_mmu_enabled(struct kvm *kvm) { return kvm->arch.tdp_mmu_enabled; }
 static inline bool is_tdp_mmu_page(struct kvm_mmu_page *sp) { return sp->tdp_mmu_page; }
 #else
-static inline void kvm_mmu_init_tdp_mmu(struct kvm *kvm) {}
+static inline bool kvm_mmu_init_tdp_mmu(struct kvm *kvm) { return false; }
 static inline void kvm_mmu_uninit_tdp_mmu(struct kvm *kvm) {}
 static inline bool is_tdp_mmu_enabled(struct kvm *kvm) { return false; }
 static inline bool is_tdp_mmu_page(struct kvm_mmu_page *sp) { return false; }

arch/x86/kvm/x86.c

@@ -10952,6 +10952,8 @@ static int memslot_rmap_alloc(struct kvm_memory_slot *slot,
 		int lpages = gfn_to_index(slot->base_gfn + npages - 1,
 					  slot->base_gfn, level) + 1;
 
+		WARN_ON(slot->arch.rmap[i]);
+
 		slot->arch.rmap[i] = kvcalloc(lpages, sz, GFP_KERNEL_ACCOUNT);
 		if (!slot->arch.rmap[i]) {
 			memslot_rmap_free(slot);
@@ -10962,6 +10964,50 @@ static int memslot_rmap_alloc(struct kvm_memory_slot *slot,
 	return 0;
 }
 
+int alloc_all_memslots_rmaps(struct kvm *kvm)
+{
+	struct kvm_memslots *slots;
+	struct kvm_memory_slot *slot;
+	int r, i;
+
+	/*
+	 * Check if memslots alreday have rmaps early before acquiring
+	 * the slots_arch_lock below.
+	 */
+	if (kvm_memslots_have_rmaps(kvm))
+		return 0;
+
+	mutex_lock(&kvm->slots_arch_lock);
+
+	/*
+	 * Read memslots_have_rmaps again, under the slots arch lock,
+	 * before allocating the rmaps
+	 */
+	if (kvm_memslots_have_rmaps(kvm)) {
+		mutex_unlock(&kvm->slots_arch_lock);
+		return 0;
+	}
+
+	for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
+		slots = __kvm_memslots(kvm, i);
+		kvm_for_each_memslot(slot, slots) {
+			r = memslot_rmap_alloc(slot, slot->npages);
+			if (r) {
+				mutex_unlock(&kvm->slots_arch_lock);
+				return r;
+			}
+		}
+	}
+
+	/*
+	 * Ensure that memslots_have_rmaps becomes true strictly after
+	 * all the rmap pointers are set.
+	 */
+	smp_store_release(&kvm->arch.memslots_have_rmaps, true);
+	mutex_unlock(&kvm->slots_arch_lock);
+	return 0;
+}
+
 static int kvm_alloc_memslot_metadata(struct kvm *kvm,
 				      struct kvm_memory_slot *slot,
 				      unsigned long npages)