[PATCH 11/15] KVM: arm64: vgic-its: Lazily allocate LPI translation cache

Oliver Upton
2024-01-24T20:49:44Z

Reusing translation cache entries within a read-side critical section is
fundamentally incompatible with an rculist. As such, we need to allocate
a new entry to replace an eviction and free the removed entry
afterwards.

Take this as an opportunity to remove the eager allocation of
translation cache entries altogether in favor of a lazy allocation model
on cache miss.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
---
arch/arm64/kvm/vgic/vgic-init.c | 3 --
arch/arm64/kvm/vgic/vgic-its.c | 86 ++++++++++++++-------------------
include/kvm/arm_vgic.h | 1 +
3 files changed, 38 insertions(+), 52 deletions(-)

diff --git a/arch/arm64/kvm/vgic/vgic-init.c b/arch/arm64/kvm/vgic/vgic-init.c
index e25672d6e846..660d5ce3b610 100644
--- a/arch/arm64/kvm/vgic/vgic-init.c
+++ b/arch/arm64/kvm/vgic/vgic-init.c
@@ -305,9 +305,6 @@ int vgic_init(struct kvm *kvm)
}
}

- if (vgic_has_its(kvm))
- vgic_lpi_translation_cache_init(kvm);
-
/*
* If we have GICv4.1 enabled, unconditionnaly request enable the
* v4 support so that we get HW-accelerated vSGIs. Otherwise, only
diff --git a/arch/arm64/kvm/vgic/vgic-its.c b/arch/arm64/kvm/vgic/vgic-its.c
index 8c026a530018..aec82d9a1b3c 100644
--- a/arch/arm64/kvm/vgic/vgic-its.c
+++ b/arch/arm64/kvm/vgic/vgic-its.c
@@ -608,12 +608,20 @@ static struct vgic_irq *vgic_its_check_cache(struct kvm *kvm, phys_addr_t db,
return irq;
}

+/* Default is 16 cached LPIs per vcpu */
+#define LPI_DEFAULT_PCPU_CACHE_SIZE 16
+
+static unsigned int vgic_its_max_cache_size(struct kvm *kvm)
+{
+ return atomic_read(&kvm->online_vcpus) * LPI_DEFAULT_PCPU_CACHE_SIZE;
+}
+
static void vgic_its_cache_translation(struct kvm *kvm, struct vgic_its *its,
u32 devid, u32 eventid,
struct vgic_irq *irq)
{
+ struct vgic_translation_cache_entry *new, *victim;
struct vgic_dist *dist = &kvm->arch.vgic;
- struct vgic_translation_cache_entry *cte;
unsigned long flags;
phys_addr_t db;

@@ -621,10 +629,11 @@ static void vgic_its_cache_translation(struct kvm *kvm, struct vgic_its *its,
if (irq->hw)
return;

- raw_spin_lock_irqsave(&dist->lpi_list_lock, flags);
+ new = victim = kzalloc(sizeof(*new), GFP_KERNEL_ACCOUNT);
+ if (!new)
+ return;

- if (unlikely(list_empty(&dist->lpi_translation_cache)))
- goto out;
+ raw_spin_lock_irqsave(&dist->lpi_list_lock, flags);

/*
* We could have raced with another CPU caching the same
@@ -635,17 +644,15 @@ static void vgic_its_cache_translation(struct kvm *kvm, struct vgic_its *its,
if (__vgic_its_check_cache(dist, db, devid, eventid))
goto out;

- /* Always reuse the last entry (LRU policy) */
- cte = list_last_entry(&dist->lpi_translation_cache,
- typeof(*cte), entry);
-
- /*
- * Caching the translation implies having an extra reference
- * to the interrupt, so drop the potential reference on what
- * was in the cache, and increment it on the new interrupt.
- */
- if (cte->irq)
- vgic_put_irq(kvm, cte->irq);
+ 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);
+ list_del(&victim->entry);
+ dist->lpi_cache_count--;
+ } else {
+ victim = NULL;
+ }

/*
* The irq refcount is guaranteed to be nonzero while holding the
@@ -654,16 +661,26 @@ static void vgic_its_cache_translation(struct kvm *kvm, struct vgic_its *its,
lockdep_assert_held(&its->its_lock);
vgic_get_irq_kref(irq);

- cte->db = db;
- cte->devid = devid;
- cte->eventid = eventid;
- cte->irq = irq;
+ new->db = db;
+ new->devid = devid;
+ new->eventid = eventid;
+ new->irq = irq;

/* Move the new translation to the head of the list */
- list_move(&cte->entry, &dist->lpi_translation_cache);
+ list_add(&new->entry, &dist->lpi_translation_cache);

out:
raw_spin_unlock_irqrestore(&dist->lpi_list_lock, flags);
+
+ /*
+ * Caching the translation implies having an extra reference
+ * to the interrupt, so drop the potential reference on what
+ * was in the cache, and increment it on the new interrupt.
+ */
+ if (victim && victim->irq)
+ vgic_put_irq(kvm, victim->irq);
+
+ kfree(victim);
}

void vgic_its_invalidate_cache(struct kvm *kvm)
@@ -1905,33 +1922,6 @@ static int vgic_register_its_iodev(struct kvm *kvm, struct vgic_its *its,
return ret;
}

-/* Default is 16 cached LPIs per vcpu */
-#define LPI_DEFAULT_PCPU_CACHE_SIZE 16
-
-void vgic_lpi_translation_cache_init(struct kvm *kvm)
-{
- struct vgic_dist *dist = &kvm->arch.vgic;
- unsigned int sz;
- int i;
-
- if (!list_empty(&dist->lpi_translation_cache))
- return;
-
- sz = atomic_read(&kvm->online_vcpus) * LPI_DEFAULT_PCPU_CACHE_SIZE;
-
- for (i = 0; i < sz; i++) {
- struct vgic_translation_cache_entry *cte;
-
- /* An allocation failure is not fatal */
- cte = kzalloc(sizeof(*cte), GFP_KERNEL_ACCOUNT);
- if (WARN_ON(!cte))
- break;
-
- INIT_LIST_HEAD(&cte->entry);
- list_add(&cte->entry, &dist->lpi_translation_cache);
- }
-}
-
void vgic_lpi_translation_cache_destroy(struct kvm *kvm)
{
struct vgic_dist *dist = &kvm->arch.vgic;
@@ -1978,8 +1968,6 @@ static int vgic_its_create(struct kvm_device *dev, u32 type)
kfree(its);
return ret;
}
-
- vgic_lpi_translation_cache_init(dev->kvm);
}

mutex_init(&its->its_lock);
diff --git a/include/kvm/arm_vgic.h b/include/kvm/arm_vgic.h
index a6f6c1583662..70490a2a300d 100644
--- a/include/kvm/arm_vgic.h
+++ b/include/kvm/arm_vgic.h
@@ -282,6 +282,7 @@ struct vgic_dist {

/* LPI translation cache */
struct list_head lpi_translation_cache;
+ unsigned int lpi_cache_count;

/* used by vgic-debug */
struct vgic_state_iter *iter;
--
2.43.0.429.g432eaa2c6b-goog