[PATCH v4 2/3] KVM: arm64: Introduce KVM_PGTABLE_WALK_SKIP_LEVEL* walk flags

[Leonardo Bras <leo.bras@arm.com>]

Add new walking flags KVM_PGTABLE_WALK_SKIP_LEVEL{0-3}.
When used, they make the kvm_pgtable_walk() skip the given level, as well
as levels higher than that.

Being able to skip levels substancially decreases the runtime of a
pagetable walk when the callback function does nothing on that level.

Signed-off-by: Leonardo Bras <leo.bras@arm.com>
---
 arch/arm64/include/asm/kvm_pgtable.h | 14 ++++++++++++++
 arch/arm64/kvm/hyp/pgtable.c         | 16 +++++++++++++---
 2 files changed, 27 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
index 41a8687938eb..5f37ceef8d63 100644
--- a/arch/arm64/include/asm/kvm_pgtable.h
+++ b/arch/arm64/include/asm/kvm_pgtable.h
@@ -311,31 +311,45 @@ typedef bool (*kvm_pgtable_force_pte_cb_t)(u64 addr, u64 end,
  * @KVM_PGTABLE_WALK_SHARED:		Indicates the page-tables may be shared
  *					with other software walkers.
  * @KVM_PGTABLE_WALK_IGNORE_EAGAIN:	Don't terminate the walk early if
  *					the walker returns -EAGAIN.
  * @KVM_PGTABLE_WALK_SKIP_BBM_TLBI:	Visit and update table entries
  *					without Break-before-make's
  *					TLB invalidation.
  * @KVM_PGTABLE_WALK_SKIP_CMO:		Visit and update table entries
  *					without Cache maintenance
  *					operations required.
+ * @KVM_PGTABLE_WALK_SKIP_LEVEL0:	Skip visiting level 0+ entries
+ * @KVM_PGTABLE_WALK_SKIP_LEVEL1:	Skip visiting level 1+ entries
+ * @KVM_PGTABLE_WALK_SKIP_LEVEL2:	Skip visiting level 2+ entries
+ * @KVM_PGTABLE_WALK_SKIP_LEVEL3:	Skip visiting level 3 entries
  */
 enum kvm_pgtable_walk_flags {
 	KVM_PGTABLE_WALK_LEAF			= BIT(0),
 	KVM_PGTABLE_WALK_TABLE_PRE		= BIT(1),
 	KVM_PGTABLE_WALK_TABLE_POST		= BIT(2),
 	KVM_PGTABLE_WALK_SHARED			= BIT(3),
 	KVM_PGTABLE_WALK_IGNORE_EAGAIN		= BIT(4),
 	KVM_PGTABLE_WALK_SKIP_BBM_TLBI		= BIT(5),
 	KVM_PGTABLE_WALK_SKIP_CMO		= BIT(6),
+	/* SKIP_LEVEL* flags should not be split nor reordered */
+	KVM_PGTABLE_WALK_SKIP_LEVEL0		= BIT(7),
+	KVM_PGTABLE_WALK_SKIP_LEVEL1		= BIT(8),
+	KVM_PGTABLE_WALK_SKIP_LEVEL2		= BIT(9),
+	KVM_PGTABLE_WALK_SKIP_LEVEL3		= BIT(10),
 };
 
+#define KVM_PGTABLE_WALK_SKIP_LEVELS 	(KVM_PGTABLE_WALK_SKIP_LEVEL0 | \
+					 KVM_PGTABLE_WALK_SKIP_LEVEL1 | \
+					 KVM_PGTABLE_WALK_SKIP_LEVEL2 | \
+					 KVM_PGTABLE_WALK_SKIP_LEVEL3 )
+
 struct kvm_pgtable_visit_ctx {
 	kvm_pte_t				*ptep;
 	kvm_pte_t				old;
 	void					*arg;
 	struct kvm_pgtable_mm_ops		*mm_ops;
 	u64					start;
 	u64					addr;
 	u64					end;
 	s8					level;
 	enum kvm_pgtable_walk_flags		flags;
diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
index 7a51f78b6446..80744478c810 100644
--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c
@@ -137,20 +137,27 @@ static bool kvm_pgtable_walk_continue(const struct kvm_pgtable_walker *walker,
 	 * Ignore the return code altogether for walkers outside a fault handler
 	 * (e.g. write protecting a range of memory) and chug along with the
 	 * page table walk.
 	 */
 	if (r == -EAGAIN)
 		return walker->flags & KVM_PGTABLE_WALK_IGNORE_EAGAIN;
 
 	return !r;
 }
 
+static bool kvm_pgtable_skip_level(s8 level, enum kvm_pgtable_walk_flags flags)
+{
+	u32 skip = FIELD_GET(KVM_PGTABLE_WALK_SKIP_LEVELS, flags);
+
+	return unlikely(skip && level >= ffs(skip) - 1);
+}
+
 static int __kvm_pgtable_walk(struct kvm_pgtable_walk_data *data,
 			      struct kvm_pgtable_mm_ops *mm_ops, kvm_pteref_t pgtable, s8 level);
 
 static inline int __kvm_pgtable_visit(struct kvm_pgtable_walk_data *data,
 				      struct kvm_pgtable_mm_ops *mm_ops,
 				      kvm_pteref_t pteref, s8 level)
 {
 	enum kvm_pgtable_walk_flags flags = data->walker->flags;
 	kvm_pte_t *ptep = kvm_dereference_pteref(data->walker, pteref);
 	struct kvm_pgtable_visit_ctx ctx = {
@@ -185,35 +192,35 @@ static inline int __kvm_pgtable_visit(struct kvm_pgtable_walk_data *data,
 	 * into a newly installed or replaced table.
 	 */
 	if (reload) {
 		ctx.old = READ_ONCE(*ptep);
 		table = kvm_pte_table(ctx.old, level);
 	}
 
 	if (!kvm_pgtable_walk_continue(data->walker, ret))
 		return ret;
 
-	if (!table) {
+	if (!table || kvm_pgtable_skip_level(level + 1, ctx.flags)) {
 		data->addr = ALIGN_DOWN(data->addr, kvm_granule_size(level));
 		data->addr += kvm_granule_size(level);
 		goto out;
 	}
 
 	childp = (kvm_pteref_t)kvm_pte_follow(ctx.old, mm_ops);
 	ret = __kvm_pgtable_walk(data, mm_ops, childp, level + 1);
 	if (!kvm_pgtable_walk_continue(data->walker, ret))
 		return ret;
 
-	if (ctx.flags & KVM_PGTABLE_WALK_TABLE_POST)
+out:
+	if (table && ctx.flags & KVM_PGTABLE_WALK_TABLE_POST)
 		ret = kvm_pgtable_visitor_cb(data, &ctx, KVM_PGTABLE_WALK_TABLE_POST);
 
-out:
 	if (kvm_pgtable_walk_continue(data->walker, ret))
 		return 0;
 
 	return ret;
 }
 
 static int __kvm_pgtable_walk(struct kvm_pgtable_walk_data *data,
 			      struct kvm_pgtable_mm_ops *mm_ops, kvm_pteref_t pgtable, s8 level)
 {
 	u32 idx;
@@ -242,20 +249,23 @@ static int _kvm_pgtable_walk(struct kvm_pgtable *pgt, struct kvm_pgtable_walk_da
 	u32 idx;
 	int ret = 0;
 	u64 limit = BIT(pgt->ia_bits);
 
 	if (data->addr > limit || data->end > limit)
 		return -ERANGE;
 
 	if (!pgt->pgd)
 		return -EINVAL;
 
+	if (kvm_pgtable_skip_level(pgt->start_level, data->walker->flags))
+		return 0;
+
 	for (idx = kvm_pgd_page_idx(pgt, data->addr); data->addr < data->end; ++idx) {
 		kvm_pteref_t pteref = &pgt->pgd[idx * PTRS_PER_PTE];
 
 		ret = __kvm_pgtable_walk(data, pgt->mm_ops, pteref, pgt->start_level);
 		if (ret)
 			break;
 	}
 
 	return ret;
 }
-- 
2.55.0