[PATCH v3 02/12] KVM: x86/mmu: bump mmu notifier count in kvm_zap_gfn_range

[Maxim Levitsky <mlevitsk@redhat.com>]

This together with previous patch, ensures that
kvm_zap_gfn_range doesn't race with page fault
running on another vcpu, and will make this page fault code
retry instead.

This is based on a patch suggested by Sean Christopherson:
https://lkml.org/lkml/2021/7/22/1025

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
---
 arch/x86/kvm/mmu/mmu.c   | 4 ++++
 include/linux/kvm_host.h | 5 +++++
 virt/kvm/kvm_main.c      | 7 +++++--
 3 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 9d78cb1c0f35..9da635e383c2 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -5640,6 +5640,8 @@ void kvm_zap_gfn_range(struct kvm *kvm, gfn_t gfn_start, gfn_t gfn_end)
 
 	write_lock(&kvm->mmu_lock);
 
+	kvm_inc_notifier_count(kvm, gfn_start, gfn_end);
+
 	if (kvm_memslots_have_rmaps(kvm)) {
 		for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
 			slots = __kvm_memslots(kvm, i);
@@ -5671,6 +5673,8 @@ void kvm_zap_gfn_range(struct kvm *kvm, gfn_t gfn_start, gfn_t gfn_end)
 	if (flush)
 		kvm_flush_remote_tlbs_with_address(kvm, gfn_start, gfn_end);
 
+	kvm_dec_notifier_count(kvm, gfn_start, gfn_end);
+
 	write_unlock(&kvm->mmu_lock);
 }
 
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 9d6b4ad407b8..962e11a73e8e 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -985,6 +985,11 @@ void kvm_mmu_free_memory_cache(struct kvm_mmu_memory_cache *mc);
 void *kvm_mmu_memory_cache_alloc(struct kvm_mmu_memory_cache *mc);
 #endif
 
+void kvm_inc_notifier_count(struct kvm *kvm, unsigned long start,
+				   unsigned long end);
+void kvm_dec_notifier_count(struct kvm *kvm, unsigned long start,
+				   unsigned long end);
+
 long kvm_arch_dev_ioctl(struct file *filp,
 			unsigned int ioctl, unsigned long arg);
 long kvm_arch_vcpu_ioctl(struct file *filp,
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index a96cbe24c688..71042cd807b3 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -608,7 +608,7 @@ static void kvm_mmu_notifier_change_pte(struct mmu_notifier *mn,
 	kvm_handle_hva_range(mn, address, address + 1, pte, kvm_set_spte_gfn);
 }
 
-static void kvm_inc_notifier_count(struct kvm *kvm, unsigned long start,
+void kvm_inc_notifier_count(struct kvm *kvm, unsigned long start,
 				   unsigned long end)
 {
 	/*
@@ -636,6 +636,7 @@ static void kvm_inc_notifier_count(struct kvm *kvm, unsigned long start,
 			max(kvm->mmu_notifier_range_end, end);
 	}
 }
+EXPORT_SYMBOL_GPL(kvm_inc_notifier_count);
 
 static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn,
 					const struct mmu_notifier_range *range)
@@ -670,7 +671,7 @@ static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn,
 	return 0;
 }
 
-static void kvm_dec_notifier_count(struct kvm *kvm, unsigned long start,
+void kvm_dec_notifier_count(struct kvm *kvm, unsigned long start,
 				   unsigned long end)
 {
 	/*
@@ -687,6 +688,8 @@ static void kvm_dec_notifier_count(struct kvm *kvm, unsigned long start,
 	 */
 	kvm->mmu_notifier_count--;
 }
+EXPORT_SYMBOL_GPL(kvm_dec_notifier_count);
+
 
 static void kvm_mmu_notifier_invalidate_range_end(struct mmu_notifier *mn,
 					const struct mmu_notifier_range *range)
-- 
2.26.3