[PATCH v2 0/2] syscall: Cleanup and improve syscall_get_arguments()

[PATCH v2 0/2] syscall: Cleanup and improve syscall_get_arguments()

[Jinjie Ruan]

Remove unused SYSCALL_MAX_ARGS and avoid memcpy() for
syscall_get_arguments() for arm64.

Jinjie Ruan (2):
  syscall.h: Remove unused SYSCALL_MAX_ARGS
  arm64: Avoid memcpy() for syscall_get_arguments()

 arch/arm/include/asm/syscall.h    |  2 --
 arch/arm64/include/asm/syscall.h  | 18 ++++++++++++------
 arch/xtensa/include/asm/syscall.h |  1 -
 3 files changed, 12 insertions(+), 9 deletions(-)

-- 
2.34.1

[PATCH v2 1/2] syscall.h: Remove unused SYSCALL_MAX_ARGS

[Jinjie Ruan]

The "SYSCALL_MAX_ARGS" appears to have been unused since
commit 32d92586629a ("syscalls: Remove start and number from
syscall_set_arguments() args"), so remove it.

Fixes: 32d92586629a ("syscalls: Remove start and number from syscall_set_arguments() args")
Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
---
 arch/arm/include/asm/syscall.h    | 2 --
 arch/arm64/include/asm/syscall.h  | 2 --
 arch/xtensa/include/asm/syscall.h | 1 -
 3 files changed, 5 deletions(-)

diff --git a/arch/arm/include/asm/syscall.h b/arch/arm/include/asm/syscall.h
index 18b102a30741..574bbcc55382 100644
--- a/arch/arm/include/asm/syscall.h
+++ b/arch/arm/include/asm/syscall.h
@@ -92,8 +92,6 @@ static inline void syscall_set_nr(struct task_struct *task,
 		(nr & __NR_SYSCALL_MASK);
 }
 
-#define SYSCALL_MAX_ARGS 7
-
 static inline void syscall_get_arguments(struct task_struct *task,
 					 struct pt_regs *regs,
 					 unsigned long *args)
diff --git a/arch/arm64/include/asm/syscall.h b/arch/arm64/include/asm/syscall.h
index 712daa90e643..f3853047c28e 100644
--- a/arch/arm64/include/asm/syscall.h
+++ b/arch/arm64/include/asm/syscall.h
@@ -77,8 +77,6 @@ static inline void syscall_set_nr(struct task_struct *task,
 	}
 }
 
-#define SYSCALL_MAX_ARGS 6
-
 static inline void syscall_get_arguments(struct task_struct *task,
 					 struct pt_regs *regs,
 					 unsigned long *args)
diff --git a/arch/xtensa/include/asm/syscall.h b/arch/xtensa/include/asm/syscall.h
index 7db3b489c8ad..bab7cdd96cbe 100644
--- a/arch/xtensa/include/asm/syscall.h
+++ b/arch/xtensa/include/asm/syscall.h
@@ -61,7 +61,6 @@ static inline void syscall_set_return_value(struct task_struct *task,
 	regs->areg[2] = (long) error ? error : val;
 }
 
-#define SYSCALL_MAX_ARGS 6
 #define XTENSA_SYSCALL_ARGUMENT_REGS {6, 3, 4, 5, 8, 9}
 
 static inline void syscall_get_arguments(struct task_struct *task,
-- 
2.34.1

[PATCH v2 2/2] arm64: Avoid memcpy() for syscall_get_arguments()

[Jinjie Ruan]

Do not use memcpy() to extract syscall arguments from struct pt_regs
but rather just perform direct assignments.

Update syscall_set_arguments() too to keep syscall_get_arguments()
and syscall_set_arguments() in sync.

With Generic Entry patch[1] and turn on audit, the performance
benchmarks from perf bench basic syscall on kunpeng920 gives roughly
a 1% performance uplift.

| Metric     | W/O this patch | With this patch | Change    |
| ---------- | -------------- | --------------- | --------- |
| Total time | 2.241 [sec]    | 2.211 [sec]     |  ↓1.36%   |
| usecs/op   | 0.224157       | 0.221146        |  ↓1.36%   |
| ops/sec    | 4,461,157      | 4,501,409       |  ↑0.9%    |

Disassembly shows that using direct assignment causes
syscall_set_arguments() to be inlined and cuts the instruction count by
five or six compared to memcpy(). Because __audit_syscall_entry() only
uses four syscall arguments, the compiler has also elided the copy of
regs->regs[4] and regs->regs[5].

Before:
<syscall_get_arguments.constprop.0>:
       aa0103e2        mov     x2, x1
       91002003        add     x3, x0, #0x8
       f9408804        ldr     x4, [x0, #272]
       f8008444        str     x4, [x2], #8
       a9409404        ldp     x4, x5, [x0, #8]
       a9009424        stp     x4, x5, [x1, #8]
       a9418400        ldp     x0, x1, [x0, #24]
       a9010440        stp     x0, x1, [x2, #16]
       f9401060        ldr     x0, [x3, #32]
       f9001040        str     x0, [x2, #32]
       d65f03c0        ret
       d503201f        nop

After:
       a9408e82        ldp     x2, x3, [x20, #8]
       2a1603e0        mov     w0, w22
       f9400e84        ldr     x4, [x20, #24]
       f9408a81        ldr     x1, [x20, #272]
       9401c4ba        bl      ffff800080215ca8 <__audit_syscall_entry>

This also aligns the implementation with x86 and RISC-V.

[1]: https://lore.kernel.org/all/20251126071446.3234218-1-ruanjinjie@huawei.com/
Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
---
v2:
- Also update syscall_set_arguments().
- Udpate the commit message.
---
 arch/arm64/include/asm/syscall.h | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/include/asm/syscall.h b/arch/arm64/include/asm/syscall.h
index f3853047c28e..5e4c7fc44f73 100644
--- a/arch/arm64/include/asm/syscall.h
+++ b/arch/arm64/include/asm/syscall.h
@@ -82,16 +82,24 @@ static inline void syscall_get_arguments(struct task_struct *task,
 					 unsigned long *args)
 {
 	args[0] = regs->orig_x0;
-	args++;
-
-	memcpy(args, &regs->regs[1], 5 * sizeof(args[0]));
+	args[1] = regs->regs[1];
+	args[2] = regs->regs[2];
+	args[3] = regs->regs[3];
+	args[4] = regs->regs[4];
+	args[5] = regs->regs[5];
 }
 
 static inline void syscall_set_arguments(struct task_struct *task,
 					 struct pt_regs *regs,
 					 const unsigned long *args)
 {
-	memcpy(&regs->regs[0], args, 6 * sizeof(args[0]));
+	regs->regs[0] = args[0];
+	regs->regs[1] = args[1];
+	regs->regs[2] = args[2];
+	regs->regs[3] = args[3];
+	regs->regs[4] = args[4];
+	regs->regs[5] = args[5];
+
 	/*
 	 * Also copy the first argument into orig_x0
 	 * so that syscall_get_arguments() would return it
-- 
2.34.1

Re: [PATCH v2 1/2] syscall.h: Remove unused SYSCALL_MAX_ARGS

[Russell King (Oracle)]

On Mon, Dec 01, 2025 at 08:06:32PM +0800, Jinjie Ruan wrote:
> The "SYSCALL_MAX_ARGS" appears to have been unused since
> commit 32d92586629a ("syscalls: Remove start and number from
> syscall_set_arguments() args"), so remove it.
> 
> Fixes: 32d92586629a ("syscalls: Remove start and number from syscall_set_arguments() args")
> Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>

Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>

Thanks!

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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