[PATCH v2 0/3] Optimize S2 page splitting

[PATCH v2 0/3] Optimize S2 page splitting

[Leonardo Bras]

While playing with dirty-bit tracking, I decided to take a look on how page
splitting works. Found out all entries are walked, even though we can infer,
for instance that:
- If a level-3 entry is walked, it means the parent level-2 entry is split
- If a split just succeeded in an table entry, it means all children nodes
  are already split

This patches' idea is to introduce new walking flags to skip pagetable
levels 0-3.

The idea of skipping child nodes was also tested, but it was marginally
slower than just skipping levels, so it was discarted.

Optimization measured on two scenarios involving eager-splitting on a
VM with 1 memslot of 16GB:
- Scenario 1: No manual protect, whole memslot split at dirty-track enable
  (KVM_SET_USER_MEMORY_REGION2 ioctl with KVM_MEM_LOG_DIRTY_PAGES)
  - Split happens only once, whole region
  - Evalutes improved batch performance of splitting
- Scenario 2: Manual protect, split happens during every dirty-bit clean
  (KVM_CLEAR_DIRTY_LOG ioctl), average for 2 iterations.
  - Split called multiple times, for smaller 64-page sections.
  - Evaluate improved performance for multiple calls

Scenario 1, improvement on dirty-track enable ioctl for the memslot:
- Memory was already split (4k pages):  -44.01% runtime (stdev 2.80%)
- THP backed memory:                    -24.66% runtime (stdev 1.21%)
- 16x1GB hugetlb memory:                -24.78% runtime (stdev 0.85%)

Scenario 2, improvement on dirty-log clean ioctl for the memslot:
- Memory was already split (4k pages):  -38.98% runtime (stdev 1.91%)
- THP backed memory:                    -25.49% runtime (stdev 0.65%)
- 16x1GB hugetlb memory:                -24.24% runtime (stdev 0.65%)

For collecting above numbers, the following script was ran in both vanilla
and patched kernels, with kernel parameter 'default_hugepagesz=1G', on an
TX2 with 32GB RAM.

--- dirty_test.sh
#!/bin/bash
filename=$(uname -r |cut -d'-' -f 4-)

run_test(){
  uname -a
  cat /proc/cmdline

  #prepare
  sudo bash -c 'echo 64 > /proc/sys/vm/nr_hugepages'

  ./dirty_log_perf_test -g -b 64G
  ./dirty_log_perf_test -g -b 64G -s anonymous_thp
  ./dirty_log_perf_test -g -b 64G -s shared_hugetlb

  ./dirty_log_perf_test -b 64G
  ./dirty_log_perf_test -b 64G -s anonymous_thp
  ./dirty_log_perf_test -b 64G -s shared_hugetlb
}

run_test 2>&1 | tee ${filename}
---

Above dirty_log_perf_test command is the standard kvm selftest found in the
kernel tree. It tested the following guest modes:
Testing guest mode: PA-bits:40,  VA-bits:48,  4K pages
Testing guest mode: PA-bits:40,  VA-bits:48, 64K pages
Testing guest mode: PA-bits:36,  VA-bits:48,  4K pages
Testing guest mode: PA-bits:36,  VA-bits:48, 64K pages

Performance numbers from above modes were used to calculate average and
stdev showed in the optimization results.

Changes since v1:
- Fixed inverted flag verification priority (Sashiko)
- Fixed incorrectly skipping POST call if level was skipped (Sashiko), and to that
- New pre-patch that changes goto-out -> return to avoid re-testing walk_continue 
v1 Link: https://lore.kernel.org/lkml/20260610202112.2695205-2-leo.bras@arm.com/

Changes since RFC:
- Changed approach from return value to walk flags (Will Deacon)
- Discarted skip_child approach (Oliver Upton)
- Measured in real hardware, and from userspace perspective (Marc Zyngier)
- Better explanation of what and how numbers were collected
RFC Link: https://lore.kernel.org/all/20260515195904.2466381-1-leo.bras@arm.com/

Thanks!
Leo

Leonardo Bras (3):
  KVM: arm64: Avoid re-testing walk_continue
  KVM: arm64: Introduce KVM_PGTABLE_WALK_SKIP_LEVEL* walk flags
  KVM: arm64: Make stage2_split_walker() skip unnecessary walks

 arch/arm64/include/asm/kvm_pgtable.h | 13 +++++++++++++
 arch/arm64/kvm/hyp/pgtable.c         | 28 +++++++++++++++++++++-------
 2 files changed, 34 insertions(+), 7 deletions(-)


base-commit: 66affa37cfac0aec061cc4bcf4a065b0c52f7e19
-- 
2.54.0

[PATCH v2 1/3] KVM: arm64: Avoid re-testing walk_continue

[Leonardo Bras]

In __kvm_pgtable_visit(), a couple tests for kvm_pgtable_walk_continue()
will 'goto out' if it should not continue. This means the same test will be
ran again before returning ret, which is unnecessary.

Return ret directly instead.
This will simplify next patch.

Signed-off-by: Leonardo Bras <leo.bras@arm.com>
---
 arch/arm64/kvm/hyp/pgtable.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
index 91a7dfad6686..4be1d51a6ac5 100644
--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c
@@ -183,32 +183,32 @@ static inline int __kvm_pgtable_visit(struct kvm_pgtable_walk_data *data,
 	 * Reload the page table after invoking the walker callback for leaf
 	 * entries or after pre-order traversal, to allow the walker to descend
 	 * 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))
-		goto out;
+		return ret;
 
 	if (!table) {
 		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))
-		goto out;
+		return ret;
 
 	if (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;
 }
-- 
2.54.0

[PATCH v2 2/3] KVM: arm64: Introduce KVM_PGTABLE_WALK_SKIP_LEVEL* walk flags

[Leonardo Bras]

Add the new walking flags that tell kvm_pgtable_walk() to skip lower levels
when walking the pagetables.

Signed-off-by: Leonardo Bras <leo.bras@arm.com>
---
 arch/arm64/include/asm/kvm_pgtable.h | 13 +++++++++++++
 arch/arm64/kvm/hyp/pgtable.c         | 19 ++++++++++++++++---
 2 files changed, 29 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
index 41a8687938eb..20c7c12e0e76 100644
--- a/arch/arm64/include/asm/kvm_pgtable.h
+++ b/arch/arm64/include/asm/kvm_pgtable.h
@@ -311,31 +311,44 @@ 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),
+	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 4be1d51a6ac5..b9a2078efc51 100644
--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c
@@ -137,20 +137,33 @@ 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 __always_inline bool kvm_pgtable_skip_level(s8 level, enum kvm_pgtable_walk_flags flags)
+{
+	flags &= KVM_PGTABLE_WALK_SKIP_LEVELS;
+
+	if (likely(!flags))
+		return false;
+
+	if (level >= (ffs(flags) - ffs(KVM_PGTABLE_WALK_SKIP_LEVELS)))
+		return true;
+
+	return false;
+}
+
 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 +198,35 @@ 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;
-- 
2.54.0

[PATCH v2 3/3] KVM: arm64: Make stage2_split_walker() skip unnecessary walks

[Leonardo Bras]

Currently, when splitting, all the child and sibling nodes will be walked,
with the walker just returning earlier if there is nothing to do. This
means all pagetable entries in the splitting range get a callback from the
walker function, even if it was a level-3 entry.

Optimize splitting by skipping all level-3 entries, as they are already the
smallest block size and can't be split any further.
(i.e. set flag KVM_PGTABLE_WALK_SKIP_LEVEL3)

kvm_mmu_split_huge_pages called in
	kvm_clear_dirty_log_protect() - manual - 64-page granularity
	kvm_mmu_split_memory_region() - no manual - all memory

Signed-off-by: Leonardo Bras <leo.bras@arm.com>
---
 arch/arm64/kvm/hyp/pgtable.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
index b9a2078efc51..9ae4d9d7ed56 100644
--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c
@@ -1565,21 +1565,22 @@ static int stage2_split_walker(const struct kvm_pgtable_visit_ctx *ctx,
 	new = kvm_init_table_pte(childp, mm_ops);
 	stage2_make_pte(ctx, new);
 	return 0;
 }
 
 int kvm_pgtable_stage2_split(struct kvm_pgtable *pgt, u64 addr, u64 size,
 			     struct kvm_mmu_memory_cache *mc)
 {
 	struct kvm_pgtable_walker walker = {
 		.cb	= stage2_split_walker,
-		.flags	= KVM_PGTABLE_WALK_LEAF,
+		.flags	= KVM_PGTABLE_WALK_LEAF |
+			  KVM_PGTABLE_WALK_SKIP_LEVEL3,
 		.arg	= mc,
 	};
 	int ret;
 
 	ret = kvm_pgtable_walk(pgt, addr, size, &walker);
 	dsb(ishst);
 	return ret;
 }
 
 int __kvm_pgtable_stage2_init(struct kvm_pgtable *pgt, struct kvm_s2_mmu *mmu,
-- 
2.54.0

Re: [PATCH v2 0/3] Optimize S2 page splitting

[Leonardo Bras]

On Thu, Jun 18, 2026 at 02:14:41PM +0100, Leonardo Bras wrote:
> While playing with dirty-bit tracking, I decided to take a look on how page
> splitting works. Found out all entries are walked, even though we can infer,
> for instance that:
> - If a level-3 entry is walked, it means the parent level-2 entry is split
> - If a split just succeeded in an table entry, it means all children nodes
>   are already split
> 
> This patches' idea is to introduce new walking flags to skip pagetable
> levels 0-3.
> 
> The idea of skipping child nodes was also tested, but it was marginally
> slower than just skipping levels, so it was discarted.
> 
> Optimization measured on two scenarios involving eager-splitting on a
> VM with 1 memslot of 16GB:
> - Scenario 1: No manual protect, whole memslot split at dirty-track enable
>   (KVM_SET_USER_MEMORY_REGION2 ioctl with KVM_MEM_LOG_DIRTY_PAGES)
>   - Split happens only once, whole region
>   - Evalutes improved batch performance of splitting
> - Scenario 2: Manual protect, split happens during every dirty-bit clean
>   (KVM_CLEAR_DIRTY_LOG ioctl), average for 2 iterations.
>   - Split called multiple times, for smaller 64-page sections.
>   - Evaluate improved performance for multiple calls
> 
> Scenario 1, improvement on dirty-track enable ioctl for the memslot:
> - Memory was already split (4k pages):  -44.01% runtime (stdev 2.80%)
> - THP backed memory:                    -24.66% runtime (stdev 1.21%)
> - 16x1GB hugetlb memory:                -24.78% runtime (stdev 0.85%)
> 
> Scenario 2, improvement on dirty-log clean ioctl for the memslot:
> - Memory was already split (4k pages):  -38.98% runtime (stdev 1.91%)
> - THP backed memory:                    -25.49% runtime (stdev 0.65%)
> - 16x1GB hugetlb memory:                -24.24% runtime (stdev 0.65%)
> 
> For collecting above numbers, the following script was ran in both vanilla
> and patched kernels, with kernel parameter 'default_hugepagesz=1G', on an
> TX2 with 32GB RAM.
> 
> --- dirty_test.sh
> #!/bin/bash
> filename=$(uname -r |cut -d'-' -f 4-)
> 
> run_test(){
>   uname -a
>   cat /proc/cmdline
> 
>   #prepare
>   sudo bash -c 'echo 64 > /proc/sys/vm/nr_hugepages'
> 
>   ./dirty_log_perf_test -g -b 64G
>   ./dirty_log_perf_test -g -b 64G -s anonymous_thp
>   ./dirty_log_perf_test -g -b 64G -s shared_hugetlb
> 
>   ./dirty_log_perf_test -b 64G
>   ./dirty_log_perf_test -b 64G -s anonymous_thp
>   ./dirty_log_perf_test -b 64G -s shared_hugetlb
> }
> 
> run_test 2>&1 | tee ${filename}
> ---

s/64G/16G/ on above script

> 
> Above dirty_log_perf_test command is the standard kvm selftest found in the
> kernel tree. It tested the following guest modes:
> Testing guest mode: PA-bits:40,  VA-bits:48,  4K pages
> Testing guest mode: PA-bits:40,  VA-bits:48, 64K pages
> Testing guest mode: PA-bits:36,  VA-bits:48,  4K pages
> Testing guest mode: PA-bits:36,  VA-bits:48, 64K pages
> 
> Performance numbers from above modes were used to calculate average and
> stdev showed in the optimization results.
> 
> Changes since v1:
> - Fixed inverted flag verification priority (Sashiko)
> - Fixed incorrectly skipping POST call if level was skipped (Sashiko), and to that
> - New pre-patch that changes goto-out -> return to avoid re-testing walk_continue 
> v1 Link: https://lore.kernel.org/lkml/20260610202112.2695205-2-leo.bras@arm.com/
> 
> Changes since RFC:
> - Changed approach from return value to walk flags (Will Deacon)
> - Discarted skip_child approach (Oliver Upton)
> - Measured in real hardware, and from userspace perspective (Marc Zyngier)
> - Better explanation of what and how numbers were collected
> RFC Link: https://lore.kernel.org/all/20260515195904.2466381-1-leo.bras@arm.com/
> 
> Thanks!
> Leo
> 
> Leonardo Bras (3):
>   KVM: arm64: Avoid re-testing walk_continue
>   KVM: arm64: Introduce KVM_PGTABLE_WALK_SKIP_LEVEL* walk flags
>   KVM: arm64: Make stage2_split_walker() skip unnecessary walks
> 
>  arch/arm64/include/asm/kvm_pgtable.h | 13 +++++++++++++
>  arch/arm64/kvm/hyp/pgtable.c         | 28 +++++++++++++++++++++-------
>  2 files changed, 34 insertions(+), 7 deletions(-)
> 
> 
> base-commit: 66affa37cfac0aec061cc4bcf4a065b0c52f7e19
> -- 
> 2.54.0
>