[PATCH v13 19/21] KVM: pfncache: check the need for invalidation under read lock first

[Paul Durrant]

From: Paul Durrant <pdurr...@amazon.com>

When processing mmu_notifier invalidations for gpc caches, pre-check for
overlap with the invalidation event while holding gpc->lock for read, and
only take gpc->lock for write if the cache needs to be invalidated.  Doing
a pre-check without taking gpc->lock for write avoids unnecessarily
contending the lock for unrelated invalidations, which is very beneficial
for caches that are heavily used (but rarely subjected to mmu_notifier
invalidations).

Signed-off-by: Paul Durrant <pdurr...@amazon.com>
Reviewed-by: David Woodhouse <d...@amazon.co.uk>
---
Cc: Sean Christopherson <sea...@google.com>
Cc: Paolo Bonzini <pbonz...@redhat.com>
Cc: David Woodhouse <dw...@infradead.org>

v13:
 - Use Sean's preferred wording for the commit comment.

v10:
 - New in this version.
---
 virt/kvm/pfncache.c | 22 +++++++++++++++++++---
 1 file changed, 19 insertions(+), 3 deletions(-)

diff --git a/virt/kvm/pfncache.c b/virt/kvm/pfncache.c
index 4e64d349b2f7..a60d8f906896 100644
--- a/virt/kvm/pfncache.c
+++ b/virt/kvm/pfncache.c
@@ -29,14 +29,30 @@ void gfn_to_pfn_cache_invalidate_start(struct kvm *kvm, 
unsigned long start,
 
        spin_lock(&kvm->gpc_lock);
        list_for_each_entry(gpc, &kvm->gpc_list, list) {
-               write_lock_irq(&gpc->lock);
+               read_lock_irq(&gpc->lock);
 
                /* Only a single page so no need to care about length */
                if (gpc->valid && !is_error_noslot_pfn(gpc->pfn) &&
                    gpc->uhva >= start && gpc->uhva < end) {
-                       gpc->valid = false;
+                       read_unlock_irq(&gpc->lock);
+
+                       /*
+                        * There is a small window here where the cache could
+                        * be modified, and invalidation would no longer be
+                        * necessary. Hence check again whether invalidation
+                        * is still necessary once the write lock has been
+                        * acquired.
+                        */
+
+                       write_lock_irq(&gpc->lock);
+                       if (gpc->valid && !is_error_noslot_pfn(gpc->pfn) &&
+                           gpc->uhva >= start && gpc->uhva < end)
+                               gpc->valid = false;
+                       write_unlock_irq(&gpc->lock);
+                       continue;
                }
-               write_unlock_irq(&gpc->lock);
+
+               read_unlock_irq(&gpc->lock);
        }
        spin_unlock(&kvm->gpc_lock);
 }
-- 
2.39.2