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KVM: pfncache: check the need for invalidation under read lock first

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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>
Link: https://lore.kernel.org/r/20240215152916.1158-20-paul@xen.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Paul Durrant 2024-02-15 15:29:14 +00:00 committed by Sean Christopherson
parent 615451d8cb
commit 9fa336e343
1 changed files with 19 additions and 3 deletions
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22
virt/kvm/pfncache.c
View file

@ -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);
}