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

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Date:   Thu, 20 May 2021 10:53:21 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 6/6] x86/syscall: use int everywhere for system call numbers

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.

Thanks,

        tglx