kvm: x86: export maximum number of mmu_page_hash collisions

Report the maximum number of mmu_page_hash collisions as a per-VM stat.
This will make it easy to identify problems with the mmu_page_hash in
the future.

Signed-off-by: David Matlack <dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

3 files changed, 20 insertions, 8 deletions

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index fc03ab1f6110..1bb1ffc0024c 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -821,6 +821,7 @@ struct kvm_vm_stat {
         ulong mmu_unsync;
         ulong remote_tlb_flush;
         ulong lpages;
+        ulong max_mmu_page_hash_collisions;
 };
 
 struct kvm_vcpu_stat {
diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 7012de4a1fed..45ee7ae88239 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -1904,17 +1904,17 @@ static void kvm_mmu_commit_zap_page(struct kvm *kvm,
  * since it has been deleted from active_mmu_pages but still can be found
  * at hast list.
  *
- * for_each_gfn_valid_sp() has skipped that kind of pages.
+ * for_each_valid_sp() has skipped that kind of pages.
  */
-#define for_each_gfn_valid_sp(_kvm, _sp, _gfn)                          \
+#define for_each_valid_sp(_kvm, _sp, _gfn)                              \
         hlist_for_each_entry(_sp,                                       \
           &(_kvm)->arch.mmu_page_hash[kvm_page_table_hashfn(_gfn)], hash_link) \
-                if ((_sp)->gfn != (_gfn) || is_obsolete_sp((_kvm), (_sp)) \
-                        || (_sp)->role.invalid) {} else
+                if (is_obsolete_sp((_kvm), (_sp)) || (_sp)->role.invalid) {    \
+                } else
 
 #define for_each_gfn_indirect_valid_sp(_kvm, _sp, _gfn)                 \
-        for_each_gfn_valid_sp(_kvm, _sp, _gfn)                          \
-                if ((_sp)->role.direct) {} else
+        for_each_valid_sp(_kvm, _sp, _gfn)                              \
+                if ((_sp)->gfn != (_gfn) || (_sp)->role.direct) {} else
 
 /* @sp->gfn should be write-protected at the call site */
 static bool __kvm_sync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
@@ -2116,6 +2116,7 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
         struct kvm_mmu_page *sp;
         bool need_sync = false;
         bool flush = false;
+        int collisions = 0;
         LIST_HEAD(invalid_list);
 
         role = vcpu->arch.mmu.base_role;
@@ -2130,7 +2131,12 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
                 quadrant &= (1 << ((PT32_PT_BITS - PT64_PT_BITS) * level)) - 1;
                 role.quadrant = quadrant;
         }
-        for_each_gfn_valid_sp(vcpu->kvm, sp, gfn) {
+        for_each_valid_sp(vcpu->kvm, sp, gfn) {
+                if (sp->gfn != gfn) {
+                        collisions++;
+                        continue;
+                }
+
                 if (!need_sync && sp->unsync)
                         need_sync = true;
 
@@ -2153,7 +2159,7 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
 
                 __clear_sp_write_flooding_count(sp);
                 trace_kvm_mmu_get_page(sp, false);
-                return sp;
+                goto out;
         }
 
         ++vcpu->kvm->stat.mmu_cache_miss;
@@ -2183,6 +2189,9 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
         trace_kvm_mmu_get_page(sp, true);
 
         kvm_mmu_flush_or_zap(vcpu, &invalid_list, false, flush);
+out:
+        if (collisions > vcpu->kvm->stat.max_mmu_page_hash_collisions)
+                vcpu->kvm->stat.max_mmu_page_hash_collisions = collisions;
         return sp;
 }
 
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index a356d8e12c2f..4aece8b0a4aa 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -190,6 +190,8 @@ struct kvm_stats_debugfs_item debugfs_entries[] = {
         { "mmu_unsync", VM_STAT(mmu_unsync) },
         { "remote_tlb_flush", VM_STAT(remote_tlb_flush) },
         { "largepages", VM_STAT(lpages) },
+        { "max_mmu_page_hash_collisions",
+                VM_STAT(max_mmu_page_hash_collisions) },
         { NULL }
 };