[PATCH 2/2] KVM: x86/mmu: Expose number of shadow MMU shadow pages as a stat

[Sean Christopherson <seanjc@google.com>]

Turn arch.n_used_mmu_pages into a stat, mmu_shadow_pages, as the number of
live shadow pages is arguably _the_ most critical datapoint when it comes
to analyzing the shadow MMU.  Before the TDP MMU came along, i.e. when the
shadow MMU was the only MMU, explicitly tracking the number of shadow pages
wasn't as interesting, because the same information could more or less be
gleaned from the pages_{1g,2m,4k} stats.  But with the TDP MMU, where the
shadow MMU is only used for nested TDP, it becomes extremely difficult, if
not impossible, to determine which SPTEs are coming from the TDP MMU, and
which are coming from the shadow MMU.

E.g. when triaging/debugging shadow MMU performance issues due to "too many
shadow pages", being able to observe that 99%+ of all shadow pages are
unsync is critical to being able to deduce that KVM is effectively leaking
shadow pages.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/include/asm/kvm_host.h |  1 +
 arch/x86/kvm/mmu/mmu.c          | 14 +++++++-------
 arch/x86/kvm/mmu/mmutrace.h     |  2 +-
 arch/x86/kvm/x86.c              |  1 +
 4 files changed, 10 insertions(+), 8 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 3886b536c8a5..be84e4d2405e 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1701,6 +1701,7 @@ struct kvm_vm_stat {
 	u64 mmu_recycled;
 	u64 mmu_cache_miss;
 	u64 mmu_unsync;
+	u64 mmu_shadow_pages;
 	union {
 		struct {
 			atomic64_t pages_4k;
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 9368a71336fe..3839aef6819b 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -1801,13 +1801,13 @@ static void kvm_mmu_check_sptes_at_free(struct kvm_mmu_page *sp)
 
 static void kvm_account_mmu_page(struct kvm *kvm, struct kvm_mmu_page *sp)
 {
-	kvm->arch.n_used_mmu_pages++;
+	kvm->stat.mmu_shadow_pages++;
 	kvm_account_pgtable_pages((void *)sp->spt, +1);
 }
 
 static void kvm_unaccount_mmu_page(struct kvm *kvm, struct kvm_mmu_page *sp)
 {
-	kvm->arch.n_used_mmu_pages--;
+	kvm->stat.mmu_shadow_pages--;
 	kvm_account_pgtable_pages((void *)sp->spt, -1);
 }
 
@@ -2833,9 +2833,9 @@ static unsigned long kvm_mmu_zap_oldest_mmu_pages(struct kvm *kvm,
 
 static inline unsigned long kvm_mmu_available_pages(struct kvm *kvm)
 {
-	if (kvm->arch.n_max_mmu_pages > kvm->arch.n_used_mmu_pages)
+	if (kvm->arch.n_max_mmu_pages > kvm->stat.mmu_shadow_pages)
 		return kvm->arch.n_max_mmu_pages -
-			kvm->arch.n_used_mmu_pages;
+			kvm->stat.mmu_shadow_pages;
 
 	return 0;
 }
@@ -2871,11 +2871,11 @@ void kvm_mmu_change_mmu_pages(struct kvm *kvm, unsigned long goal_nr_mmu_pages)
 {
 	write_lock(&kvm->mmu_lock);
 
-	if (kvm->arch.n_used_mmu_pages > goal_nr_mmu_pages) {
-		kvm_mmu_zap_oldest_mmu_pages(kvm, kvm->arch.n_used_mmu_pages -
+	if (kvm->stat.mmu_shadow_pages > goal_nr_mmu_pages) {
+		kvm_mmu_zap_oldest_mmu_pages(kvm, kvm->stat.mmu_shadow_pages -
 						  goal_nr_mmu_pages);
 
-		goal_nr_mmu_pages = kvm->arch.n_used_mmu_pages;
+		goal_nr_mmu_pages = kvm->stat.mmu_shadow_pages;
 	}
 
 	kvm->arch.n_max_mmu_pages = goal_nr_mmu_pages;
diff --git a/arch/x86/kvm/mmu/mmutrace.h b/arch/x86/kvm/mmu/mmutrace.h
index fa01719baf8d..8354d9f39777 100644
--- a/arch/x86/kvm/mmu/mmutrace.h
+++ b/arch/x86/kvm/mmu/mmutrace.h
@@ -303,7 +303,7 @@ TRACE_EVENT(
 
 	TP_fast_assign(
 		__entry->mmu_valid_gen = kvm->arch.mmu_valid_gen;
-		__entry->mmu_used_pages = kvm->arch.n_used_mmu_pages;
+		__entry->mmu_used_pages = kvm->stat.mmu_shadow_pages;
 	),
 
 	TP_printk("kvm-mmu-valid-gen %u used_pages %x",
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index cd68a5bad0c6..e4cbecaa105d 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -244,6 +244,7 @@ const struct kvm_stats_desc kvm_vm_stats_desc[] = {
 	STATS_DESC_COUNTER(VM, mmu_recycled),
 	STATS_DESC_COUNTER(VM, mmu_cache_miss),
 	STATS_DESC_ICOUNTER(VM, mmu_unsync),
+	STATS_DESC_ICOUNTER(VM, mmu_shadow_pages),
 	STATS_DESC_ICOUNTER(VM, pages_4k),
 	STATS_DESC_ICOUNTER(VM, pages_2m),
 	STATS_DESC_ICOUNTER(VM, pages_1g),
-- 
2.54.0.1032.g2f8565e1d1-goog