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

Jinjie Ruan
2025-12-01T12:06:50Z

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