[PATCH] KVM: arm64: vgic-its: Fix O(C*I) loop in vgic_its_free_collection_list

[Jing Zhang <jingzhangos@google.com>]

When destroying the vgic-its collection list,
vgic_its_free_collection_list() iterates over every collection and for
each, calls vgic_its_free_collection(). This function walks every
Interrupt Translation Entry (ITE) across all devices via
for_each_lpi_its() to nullify the collection pointer.

A guest can allocate up to 65536 collections and hundreds of thousands
of ITEs. By clearing GITS_CTLR.Enable and writing Valid=0 to
GITS_BASER1, the guest can trigger this teardown path from a single MMIO
exit. The resulting O(Collections * ITEs) nested loop executes billions
of iterations without a single cond_resched(). This pins a physical CPU
and stalls RCU grace periods for seconds or minutes on PREEMPT_NONE
kernels.

Fix this by replacing the O(Collections * ITEs) teardown with an
O(Collections + ITEs) pass. Since the entire collection list is being
freed, we can safely bulk-clear the collection pointers from all ITEs in
a single pass, and then free all the collections in a second pass.

Signed-off-by: Jing Zhang <jingzhangos@google.com>
---
 arch/arm64/kvm/vgic/vgic-its.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kvm/vgic/vgic-its.c b/arch/arm64/kvm/vgic/vgic-its.c
index 36ab3e4929154..a8e819fe97898 100644
--- a/arch/arm64/kvm/vgic/vgic-its.c
+++ b/arch/arm64/kvm/vgic/vgic-its.c
@@ -1133,9 +1133,21 @@ static void vgic_its_free_device_list(struct kvm *kvm, struct vgic_its *its)
 static void vgic_its_free_collection_list(struct kvm *kvm, struct vgic_its *its)
 {
 	struct its_collection *cur, *temp;
+	struct its_device *device;
+	struct its_ite *ite;
 
-	list_for_each_entry_safe(cur, temp, &its->collection_list, coll_list)
-		vgic_its_free_collection(its, cur->collection_id);
+	/*
+	 * Bulk-clear the collection pointers for all ITEs.
+	 * This transforms the teardown complexity from O(Collections * ITEs)
+	 * to O(Collections + ITEs), avoiding guest-triggered host RCU stalls.
+	 */
+	for_each_lpi_its(device, ite, its)
+		ite->collection = NULL;
+
+	list_for_each_entry_safe(cur, temp, &its->collection_list, coll_list) {
+		list_del(&cur->coll_list);
+		kfree(cur);
+	}
 }
 
 /* Must be called with its_lock mutex held */
-- 
2.55.0.737.g08866a6d13-goog