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

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Date:	Wed, 25 Feb 2015 09:43:12 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	"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, 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.

-- Steve

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