[PATCH v2] arm64: clear_page[s] using memset

[Linus Walleij <linusw@kernel.org>]

There is no need to try to second-guess the compiler when
clearing memory. Just call memset() like everyone else.

Since memset() already has an architecture-local MOPS
optimization, we do not need to do anything else to preserve
the MOPS optimization.

While at it, implement the shorthand for directly calling
the new prototype clear_pages() for larger page chunks.

No performance regressions can be seen, the fastpath
benchmarks differences are in the noise.

Usama Anjum tested next-20260821 with one warm-up and three repeats
in four sessions, for a total of 12 measured runs. The commands were:

  perf bench mem memset -k 1GB -f default -s 16GB
  perf bench mem mmap -p 1GB -f demand -s 32GB -l 5
  perf bench mem mmap -p 4KB -f demand -s 32GB -l 5

The results were:

aws-m7g.metal:
  Benchmark    Base bytes/sec   Change with patch
  memset 1GB   63932542232.12                1.92%
  mmap 1GB     63272579168.03               -0.57%
  mmap 4KB     49692830849.48               -1.11%

cesw-aarch64-ampereone-1s-a192-32x:
  Benchmark    Base bytes/sec   Change with patch
  memset 1GB   33895562998.71                0.25%
  mmap 1GB     34338454210.17                1.16%
  mmap 4KB     25687107580.90               -0.85%

Assisted-by: LLM
Suggested-by: Will Deacon <will@kernel.org>
Tested-by: Usama Anjum <usama.anjum@arm.com>
Link: https://lore.kernel.org/linux-arm-kernel/20260303-aarch64-clear-pages-v1-1-ad0c3ee9a555@kernel.org/
Link: https://lore.kernel.org/linux-arm-kernel/67ab6c51-4511-46ab-931b-a4323551147a@arm.com/
Signed-off-by: Linus Walleij <linusw@kernel.org>
---
Changes in v2:
- Rebased on v7.3-rc1
- Include Usama Anjum's performance results and Tested-by tag.
- Link to v1: https://lore.kernel.org/r/20260306-aarch64-clear-pages-c-v1-1-77c1bb0f1c21@kernel.org
---
 arch/arm64/include/asm/page.h    | 13 +++++++++-
 arch/arm64/kernel/image-vars.h   |  1 -
 arch/arm64/kvm/hyp/nvhe/Makefile |  2 +-
 arch/arm64/lib/Makefile          |  2 +-
 arch/arm64/lib/clear_page.S      | 53 ----------------------------------------
 5 files changed, 14 insertions(+), 57 deletions(-)

diff --git a/arch/arm64/include/asm/page.h b/arch/arm64/include/asm/page.h
index 58200de8a221..b2eaa63d9056 100644
--- a/arch/arm64/include/asm/page.h
+++ b/arch/arm64/include/asm/page.h
@@ -13,6 +13,7 @@
 #ifndef __ASSEMBLER__
 
 #include <linux/personality.h> /* for READ_IMPLIES_EXEC */
+#include <linux/string.h> /* for memset() */
 #include <linux/types.h> /* for gfp_t */
 #include <asm/pgtable-types.h>
 
@@ -20,7 +21,17 @@ struct page;
 struct vm_area_struct;
 
 extern void copy_page(void *to, const void *from);
-extern void clear_page(void *to);
+
+static inline void clear_pages(void *addr, unsigned int npages)
+{
+	memset(addr, 0, npages * PAGE_SIZE);
+}
+#define clear_pages clear_pages
+
+static inline void clear_page(void *addr)
+{
+	clear_pages(addr, 1);
+}
 
 void copy_user_highpage(struct page *to, struct page *from,
 			unsigned long vaddr, struct vm_area_struct *vma);
diff --git a/arch/arm64/kernel/image-vars.h b/arch/arm64/kernel/image-vars.h
index 14beb7b9d304..facda8137d9b 100644
--- a/arch/arm64/kernel/image-vars.h
+++ b/arch/arm64/kernel/image-vars.h
@@ -119,7 +119,6 @@ KVM_NVHE_ALIAS(__start___kvm_ex_table);
 KVM_NVHE_ALIAS(__stop___kvm_ex_table);
 
 /* Position-independent library routines */
-KVM_NVHE_ALIAS_HYP(clear_page, __pi_clear_page);
 KVM_NVHE_ALIAS_HYP(copy_page, __pi_copy_page);
 KVM_NVHE_ALIAS_HYP(memcpy, __pi_memcpy);
 KVM_NVHE_ALIAS_HYP(memset, __pi_memset);
diff --git a/arch/arm64/kvm/hyp/nvhe/Makefile b/arch/arm64/kvm/hyp/nvhe/Makefile
index f57450ebcb49..2ed489bf544e 100644
--- a/arch/arm64/kvm/hyp/nvhe/Makefile
+++ b/arch/arm64/kvm/hyp/nvhe/Makefile
@@ -17,7 +17,7 @@ ccflags-y += -fno-stack-protector	\
 hostprogs := gen-hyprel
 HOST_EXTRACFLAGS += -I$(objtree)/include
 
-lib-objs := clear_page.o copy_page.o memcpy.o memset.o tishift.o
+lib-objs := copy_page.o memcpy.o memset.o tishift.o
 lib-objs := $(addprefix ../../../lib/, $(lib-objs))
 
 CFLAGS_switch.nvhe.o += -Wno-override-init
diff --git a/arch/arm64/lib/Makefile b/arch/arm64/lib/Makefile
index b33e1ca4a781..38ffd359123c 100644
--- a/arch/arm64/lib/Makefile
+++ b/arch/arm64/lib/Makefile
@@ -5,7 +5,7 @@ KCSAN_SANITIZE_delay.o := n
 
 lib-y		:= clear_user.o delay.o copy_from_user.o		\
 		   copy_to_user.o copy_page.o				\
-		   clear_page.o csum.o insn.o memchr.o memcpy.o		\
+		   csum.o insn.o memchr.o memcpy.o			\
 		   memset.o memcmp.o strcmp.o strncmp.o strlen.o	\
 		   strnlen.o strchr.o strrchr.o tishift.o
 
diff --git a/arch/arm64/lib/clear_page.S b/arch/arm64/lib/clear_page.S
deleted file mode 100644
index bd6f7d5eb6eb..000000000000
--- a/arch/arm64/lib/clear_page.S
+++ /dev/null
@@ -1,53 +0,0 @@
-/* SPDX-License-Identifier: GPL-2.0-only */
-/*
- * Copyright (C) 2012 ARM Ltd.
- */
-
-#include <linux/linkage.h>
-#include <linux/const.h>
-#include <asm/assembler.h>
-#include <asm/page.h>
-
-/*
- * Clear page @dest
- *
- * Parameters:
- *	x0 - dest
- */
-SYM_FUNC_START(__pi_clear_page)
-#ifdef CONFIG_AS_HAS_MOPS
-	.arch_extension mops
-alternative_if_not ARM64_HAS_MOPS
-	b	.Lno_mops
-alternative_else_nop_endif
-
-	mov	x1, #PAGE_SIZE
-	setpn	[x0]!, x1!, xzr
-	setmn	[x0]!, x1!, xzr
-	seten	[x0]!, x1!, xzr
-	ret
-.Lno_mops:
-#endif
-	mrs	x1, dczid_el0
-	tbnz	x1, #4, 2f	/* Branch if DC ZVA is prohibited */
-	and	w1, w1, #0xf
-	mov	x2, #4
-	lsl	x1, x2, x1
-
-1:	dc	zva, x0
-	add	x0, x0, x1
-	tst	x0, #(PAGE_SIZE - 1)
-	b.ne	1b
-	ret
-
-2:	stnp	xzr, xzr, [x0]
-	stnp	xzr, xzr, [x0, #16]
-	stnp	xzr, xzr, [x0, #32]
-	stnp	xzr, xzr, [x0, #48]
-	add	x0, x0, #64
-	tst	x0, #(PAGE_SIZE - 1)
-	b.ne	2b
-	ret
-SYM_FUNC_END(__pi_clear_page)
-SYM_FUNC_ALIAS(clear_page, __pi_clear_page)
-EXPORT_SYMBOL(clear_page)

---
base-commit: cee9395acd8043be0644b25c34bfa86623f2b935
change-id: 20260305-aarch64-clear-pages-c-590dae98c333

Best regards,
--  
Linus Walleij <linusw@kernel.org>