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

Paul Durrant
2024-02-15T15:44:32Z

From: Paul Durrant <pdurrant@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 <pdurrant@amazon.com>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
---
Cc: Sean Christopherson <seanjc@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: David Woodhouse <dwmw2@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