[PATCH 12/15] KVM: arm64: vgic-its: Pick cache victim based on usage count

Oliver Upton
2024-01-24T20:49:46Z

To date the translation cache LRU policy relies on the ordering of the
linked-list to pick the victim, as entries are moved to the head of the
list on every cache hit. These sort of transformations are incompatible
with an rculist, necessitating a different strategy for recording usage
in-place.

Count the number of cache hits since the last translation cache miss for
every entry. The preferences for selecting a victim are as follows:

- Invalid entries over valid entries

- Valid entry with the lowest usage count

- In the case of a tie, pick the entry closest to the tail (oldest)

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
---
arch/arm64/kvm/vgic/vgic-its.c | 42 ++++++++++++++++++++++++++--------
1 file changed, 32 insertions(+), 10 deletions(-)

diff --git a/arch/arm64/kvm/vgic/vgic-its.c b/arch/arm64/kvm/vgic/vgic-its.c
index aec82d9a1b3c..ed0c6c333a6c 100644
--- a/arch/arm64/kvm/vgic/vgic-its.c
+++ b/arch/arm64/kvm/vgic/vgic-its.c
@@ -154,6 +154,7 @@ struct vgic_translation_cache_entry {
u32 devid;
u32 eventid;
struct vgic_irq *irq;
+ atomic64_t usage_count;
};

/**
@@ -577,13 +578,7 @@ static struct vgic_irq *__vgic_its_check_cache(struct vgic_dist *dist,
cte->eventid != eventid)
continue;

- /*
- * Move this entry to the head, as it is the most
- * recently used.
- */
- if (!list_is_first(&cte->entry, &dist->lpi_translation_cache))
- list_move(&cte->entry, &dist->lpi_translation_cache);
-
+ atomic64_inc(&cte->usage_count);
return cte->irq;
}

@@ -616,6 +611,30 @@ static unsigned int vgic_its_max_cache_size(struct kvm *kvm)
return atomic_read(&kvm->online_vcpus) * LPI_DEFAULT_PCPU_CACHE_SIZE;
}

+static struct vgic_translation_cache_entry *vgic_its_cache_victim(struct vgic_dist *dist)
+{
+ struct vgic_translation_cache_entry *cte, *victim = NULL;
+ u64 min, tmp;
+
+ /*
+ * Find the least used cache entry since the last cache miss, preferring
+ * older entries in the case of a tie. Note that usage accounting is
+ * deliberately non-atomic, so this is all best-effort.
+ */
+ list_for_each_entry(cte, &dist->lpi_translation_cache, entry) {
+ if (!cte->irq)
+ return cte;
+
+ tmp = atomic64_xchg_relaxed(&cte->usage_count, 0);
+ if (!victim || tmp <= min) {
+ victim = cte;
+ min = tmp;
+ }
+ }
+
+ return victim;
+}
+
static void vgic_its_cache_translation(struct kvm *kvm, struct vgic_its *its,
u32 devid, u32 eventid,
struct vgic_irq *irq)
@@ -645,9 +664,12 @@ static void vgic_its_cache_translation(struct kvm *kvm, struct vgic_its *its,
goto out;

if (dist->lpi_cache_count >= vgic_its_max_cache_size(kvm)) {
- /* Always reuse the last entry (LRU policy) */
- victim = list_last_entry(&dist->lpi_translation_cache,
- typeof(*cte), entry);
+ victim = vgic_its_cache_victim(dist);
+ if (WARN_ON_ONCE(!victim)) {
+ victim = new;
+ goto out;
+ }
+
list_del(&victim->entry);
dist->lpi_cache_count--;
} else {
--
2.43.0.429.g432eaa2c6b-goog