[PATCH 09/14] KVM: arm64: Free removed stage-2 tables in RCU callback

[Oliver Upton]

There is no real urgency to free a stage-2 subtree that was pruned.
Nonetheless, KVM does the tear down in the stage-2 fault path while
holding the MMU lock.

Free removed stage-2 subtrees after an RCU grace period. To guarantee
all stage-2 table pages are freed before killing a VM, add an
rcu_barrier() to the flush path.

Signed-off-by: Oliver Upton <oliver.upton at linux.dev>
---
 arch/arm64/kvm/mmu.c | 35 ++++++++++++++++++++++++++++++++++-
 1 file changed, 34 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 91521f4aab97..265951c05879 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -97,6 +97,38 @@ static void *stage2_memcache_zalloc_page(void *arg)
 	return kvm_mmu_memory_cache_alloc(mc);
 }
 
+#define STAGE2_PAGE_PRIVATE_LEVEL_MASK	GENMASK_ULL(2, 0)
+
+static inline unsigned long stage2_page_private(u32 level, void *arg)
+{
+	unsigned long pvt = (unsigned long)arg;
+
+	BUILD_BUG_ON(KVM_PGTABLE_MAX_LEVELS > STAGE2_PAGE_PRIVATE_LEVEL_MASK);
+	WARN_ON_ONCE(pvt & STAGE2_PAGE_PRIVATE_LEVEL_MASK);
+
+	return pvt | level;
+}
+
+static void stage2_free_removed_table_rcu_cb(struct rcu_head *head)
+{
+	struct page *page = container_of(head, struct page, rcu_head);
+	unsigned long pvt = page_private(page);
+	void *arg = (void *)(pvt & ~STAGE2_PAGE_PRIVATE_LEVEL_MASK);
+	u32 level = (u32)(pvt & STAGE2_PAGE_PRIVATE_LEVEL_MASK);
+	void *pgtable = page_to_virt(page);
+
+	kvm_pgtable_stage2_free_removed(pgtable, level, arg);
+}
+
+static void stage2_free_removed_table(void *pgtable, u32 level, void *arg)
+{
+	unsigned long pvt = stage2_page_private(level, arg);
+	struct page *page = virt_to_page(pgtable);
+
+	set_page_private(page, (unsigned long)pvt);
+	call_rcu(&page->rcu_head, stage2_free_removed_table_rcu_cb);
+}
+
 static void *kvm_host_zalloc_pages_exact(size_t size)
 {
 	return alloc_pages_exact(size, GFP_KERNEL_ACCOUNT | __GFP_ZERO);
@@ -627,7 +659,7 @@ static struct kvm_pgtable_mm_ops kvm_s2_mm_ops = {
 	.zalloc_page		= stage2_memcache_zalloc_page,
 	.zalloc_pages_exact	= kvm_host_zalloc_pages_exact,
 	.free_pages_exact	= free_pages_exact,
-	.free_removed_table	= kvm_pgtable_stage2_free_removed,
+	.free_removed_table	= stage2_free_removed_table,
 	.get_page		= kvm_host_get_page,
 	.put_page		= kvm_host_put_page,
 	.page_count		= kvm_host_page_count,
@@ -770,6 +802,7 @@ void kvm_free_stage2_pgd(struct kvm_s2_mmu *mmu)
 	if (pgt) {
 		kvm_pgtable_stage2_destroy(pgt);
 		kfree(pgt);
+		rcu_barrier();
 	}
 }
 
-- 
2.37.2.672.g94769d06f0-goog