linux-kernel - Re: [PATCH 1/4] x86: entry.S: tidy up several suboptimal insns

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Date:	Wed, 25 Feb 2015 16:40:49 +0100
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Ingo Molnar <mingo@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4] x86: entry.S: tidy up several suboptimal insns

On Wed, Feb 25, 2015 at 3:43 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Wed, 25 Feb 2015 10:20:43 +0100 Ingo Molnar <mingo@...nel.org> wrote:
>> But, if we do that, we can do even better, and also do an
>> optimization of the 64-bit entry path as well: we could
>> simply mask RAX with 0x3ff and not do a compare. Pad the
>> syscall table up to 0x400 (1024) entries and fill in the
>> table with sys_ni syscall entries.
>>
>> This is valid on 64-bit and 32-bit kernels as well, and it
>> allows the removal of a compare from the syscall entry
>> path, at the cost of a couple of kilobytes of unused
>> syscall table.
>>
>> The downside would be that if we ever grow past 1024
>> syscall entries we'll be in trouble if new userspace calls
>> syscall 513 on an old kernel and gets syscall 1.
>
> What if we test against ~0x3ff and jump to sys_ni if anything is set.
> This would still work with new userspace calls on older kernels.

That would require a branch insn. The whole idea of masking
was merely to avoid that branch. If you need a branch,
then you can as well just retain current code.
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