Re: [PATCH v4 6/6] x86/syscall: use int everywhere for system call numbers

From: H. Peter Anvin
Date: Fri May 21 2021 - 17:37:05 EST




On 5/20/21 1:53 AM, Thomas Gleixner wrote:
On Tue, May 18 2021 at 12:13, H. Peter Anvin wrote:
+static __always_inline bool do_syscall_x64(struct pt_regs *regs, int nr)
+{
+ /*
+ * Convert negative numbers to very high and thus out of range
+ * numbers for comparisons. Use unsigned long to slightly
+ * improve the array_index_nospec() generated code.

How is that actually improving the generated code?

unsigned long:

104: 48 81 fa bf 01 00 00 cmp $0x1bf,%rdx
10b: 48 19 c0 sbb %rax,%rax
10e: 48 21 c2 and %rax,%rdx
111: 48 89 df mov %rbx,%rdi
114: 48 8b 04 d5 00 00 00 mov 0x0(,%rdx,8),%rax
11b: 00
11c: e8 00 00 00 00 callq 121 <do_syscall_64+0x41>

unsigned int:

f1: 48 81 fa bf 01 00 00 cmp $0x1bf,%rdx
f8: 48 19 d2 sbb %rdx,%rdx
fb: 21 d0 and %edx,%eax
fd: 48 89 df mov %rbx,%rdi
100: 48 8b 04 c5 00 00 00 mov 0x0(,%rax,8),%rax
107: 00
108: e8 00 00 00 00 callq 10d <do_syscall_64+0x3d>

Text size increases with that unsigned long cast.

I must be missing something.


"unsigned long" gave slightly better code than "int", but as you correctly point out here, "unsigned int" is even better.

Thanks for catching that.

-hpa