[RFC PATCH 6/6] KVM: arm64: Create a fast stage-2 unmap path

[Raghavendra Rao Ananta <rananta@google.com>]

The current implementation of the stage-2 unmap walker
traverses the entire page-table to clear and flush the TLBs
for each entry. This could be very expensive if the VM
is not backed by hugepages. The unmap operation could be
made efficient by disconnecting the table at the very
top (level at which the largest block mapping can be hosted)
and do the rest of the unmapping using free_removed_table().
If the system supports FEAT_TLBIRANGE, flush the entire range
that has been disconnected from the rest of the page-table.

Suggested-by: Ricardo Koller <ricarkol@google.com>
Signed-off-by: Raghavendra Rao Ananta <rananta@google.com>
---
 arch/arm64/kvm/hyp/pgtable.c | 44 ++++++++++++++++++++++++++++++++++++
 1 file changed, 44 insertions(+)

diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
index 099032bb01bce..7bcd898de2805 100644
--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c
@@ -1021,6 +1021,49 @@ static int stage2_unmap_walker(const struct kvm_pgtable_visit_ctx *ctx,
 	return 0;
 }
 
+/*
+ * The fast walker executes only if the unmap size is exactly equal to the
+ * largest block mapping supported (i.e. at KVM_PGTABLE_MIN_BLOCK_LEVEL),
+ * such that the underneath hierarchy at KVM_PGTABLE_MIN_BLOCK_LEVEL can
+ * be disconnected from the rest of the page-table without the need to
+ * traverse all the PTEs, at all the levels, and unmap each and every one
+ * of them. The disconnected table can be freed using free_removed_table().
+ */
+static int fast_stage2_unmap_walker(const struct kvm_pgtable_visit_ctx *ctx,
+			       enum kvm_pgtable_walk_flags visit)
+{
+	struct kvm_pgtable_mm_ops *mm_ops = ctx->mm_ops;
+	kvm_pte_t *childp = kvm_pte_follow(ctx->old, mm_ops);
+	struct kvm_s2_mmu *mmu = ctx->arg;
+
+	if (!kvm_pte_valid(ctx->old) || ctx->level != KVM_PGTABLE_MIN_BLOCK_LEVEL)
+		return 0;
+
+	if (!stage2_try_break_pte(ctx, mmu, 0))
+		return -EAGAIN;
+
+	/*
+	 * Gain back a reference for stage2_unmap_walker() to free
+	 * this table entry from KVM_PGTABLE_MIN_BLOCK_LEVEL - 1.
+	 */
+	mm_ops->get_page(ctx->ptep);
+
+	mm_ops->free_removed_table(childp, ctx->level);
+	return 0;
+}
+
+static void kvm_pgtable_try_fast_stage2_unmap(struct kvm_pgtable *pgt, u64 addr, u64 size)
+{
+	struct kvm_pgtable_walker walker = {
+		.cb	= fast_stage2_unmap_walker,
+		.arg	= pgt->mmu,
+		.flags	= KVM_PGTABLE_WALK_TABLE_PRE,
+	};
+
+	if (size == kvm_granule_size(KVM_PGTABLE_MIN_BLOCK_LEVEL))
+		kvm_pgtable_walk(pgt, addr, size, &walker);
+}
+
 int kvm_pgtable_stage2_unmap(struct kvm_pgtable *pgt, u64 addr, u64 size)
 {
 	struct kvm_pgtable_walker walker = {
@@ -1029,6 +1072,7 @@ int kvm_pgtable_stage2_unmap(struct kvm_pgtable *pgt, u64 addr, u64 size)
 		.flags	= KVM_PGTABLE_WALK_LEAF | KVM_PGTABLE_WALK_TABLE_POST,
 	};
 
+	kvm_pgtable_try_fast_stage2_unmap(pgt, addr, size);
 	return kvm_pgtable_walk(pgt, addr, size, &walker);
 }
 
-- 
2.39.0.314.g84b9a713c41-goog


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