arm64: avoid memcpy() for syscall_get_arguments()

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, torvalds#272]
       f8008444        str     x4, [x2], torvalds#8
       a9409404        ldp     x4, x5, [x0, torvalds#8]
       a9009424        stp     x4, x5, [x1, torvalds#8]
       a9418400        ldp     x0, x1, [x0, torvalds#24]
       a9010440        stp     x0, x1, [x2, torvalds#16]
       f9401060        ldr     x0, [x3, torvalds#32]
       f9001040        str     x0, [x2, torvalds#32]
       d65f03c0        ret
       d503201f        nop

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

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

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lore.kernel.org/all/[email protected]/ [1]
Signed-off-by: Jinjie Ruan <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Charlie Jenkins <[email protected]>
Cc: Christian Zankel <[email protected]>
Cc: "Dmitry V. Levin" <[email protected]>
Cc: Helge Deller <[email protected]>
Cc: Maciej W. Rozycki <[email protected]>
Cc: Marc Rutland <[email protected]>
Cc: Max Filippov <[email protected]>
Cc: Russell King (Oracle) <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Thomas Gleinxer <[email protected]>
Cc: Will Deacon <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Jinjie Ruan

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