[PATCH 2/5] KVM: pfncache: Wait for in-progress invalidations to complete during refresh

[Sean Christopherson]

When refreshing a gpc, wait for in-progress invalidations to complete
before reading the sequence counter and resolving the pfn.  Resolving the
pfn when there is an in-progress invalidation is worse than pointless, as
the pfn is guaranteed to be discarded, and trying to resolve the pfn could
contended for resources, e.g. mmap_lock, and make the invalidation and
thus refresh take longer to complete.

Suggested-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 virt/kvm/pfncache.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/virt/kvm/pfncache.c b/virt/kvm/pfncache.c
index 4afbc1262e3f..957f739227ab 100644
--- a/virt/kvm/pfncache.c
+++ b/virt/kvm/pfncache.c
@@ -194,6 +194,18 @@ static kvm_pfn_t hva_to_pfn_retry(struct gfn_to_pfn_cache *gpc)
 			cond_resched();
 		}
 
+		/*
+		 * Wait for in-progress invalidations to complete if the user
+		 * already being invalidated.  Unlike the page fault path, this
+		 * task _must_ complete the refresh, i.e. there's no value in
+		 * trying to race ahead in the hope that a different task makes
+		 * the cache valid.
+		 */
+		while (READ_ONCE(gpc->kvm->mn_active_invalidate_count)) {
+			if (!cond_resched())
+				cpu_relax();
+		}
+
 		mmu_seq = gpc->kvm->mmu_invalidate_seq;
 		smp_rmb();
 
-- 
2.47.0.rc1.288.g06298d1525-goog


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