From: "H. Peter Anvin" <hpa@zytor.com>
To: Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Andy Lutomirski <luto@kernel.org>,
Borislav Petkov <bp@alien8.de>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v4 6/6] x86/syscall: use int everywhere for system call numbers
Date: Fri, 21 May 2021 14:36:37 -0700 [thread overview]
Message-ID: <e43577cf-ab35-1de2-818a-ccc2e2fb99b8@zytor.com> (raw)
In-Reply-To: <87zgwpbxby.ffs@nanos.tec.linutronix.de>
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
next prev parent reply other threads:[~2021-05-21 21:37 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-18 19:12 [PATCH v4 0/6] x86/syscall: use int for x86-64 system calls H. Peter Anvin
2021-05-18 19:12 ` [PATCH v4 1/6] x86/syscall: update and extend selftest syscall_numbering_64 H. Peter Anvin
2021-05-20 13:23 ` [tip: x86/entry] selftests/x86/syscall: Update and extend syscall_numbering_64 tip-bot2 for H. Peter Anvin (Intel)
2021-05-18 19:12 ` [PATCH v4 2/6] x86/syscall: simplify message reporting in syscall_numbering.c H. Peter Anvin
2021-05-20 13:23 ` [tip: x86/entry] selftests/x86/syscall: Simplify message reporting in syscall_numbering tip-bot2 for H. Peter Anvin (Intel)
2021-05-18 19:13 ` [PATCH v4 3/6] x86/syscall: add tests under ptrace to syscall_numbering.c H. Peter Anvin
2021-05-20 13:23 ` [tip: x86/entry] selftests/x86/syscall: Add tests under ptrace to syscall_numbering_64 tip-bot2 for H. Peter Anvin (Intel)
2021-05-18 19:13 ` [PATCH v4 4/6] x86/syscall: sign-extend system calls on entry to int H. Peter Anvin
2021-05-20 13:23 ` [tip: x86/entry] x86/entry/64: Sign-extend " tip-bot2 for H. Peter Anvin (Intel)
2021-05-18 19:13 ` [PATCH v4 5/6] x86/syscall: treat out of range and gap system calls the same H. Peter Anvin
2021-05-20 13:23 ` [tip: x86/entry] x86/entry: Treat " tip-bot2 for H. Peter Anvin (Intel)
2021-05-18 19:13 ` [PATCH v4 6/6] x86/syscall: use int everywhere for system call numbers H. Peter Anvin
2021-05-20 8:53 ` Thomas Gleixner
2021-05-21 21:36 ` H. Peter Anvin [this message]
2021-05-22 13:19 ` David Laight
2021-05-25 8:13 ` [tip: x86/entry] x86/entry: Use " tip-bot2 for H. Peter Anvin (Intel)
2021-05-19 11:29 ` [PATCH v4 0/6] x86/syscall: use int for x86-64 system calls Ingo Molnar
2021-05-19 16:17 ` H. Peter Anvin
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