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

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Date:   Fri, 21 May 2021 14:36:37 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>
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 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