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

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Message-ID: <20150225110129.5d099cd8@gandalf.local.home>
Date:	Wed, 25 Feb 2015 11:01:29 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Denys Vlasenko <vda.linux@...glemail.com>
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, 25 Feb 2015 16:40:49 +0100
Denys Vlasenko <vda.linux@...glemail.com> wrote:

> >> 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.

I'm just curious, do all these micro optimizations have any real impact
on real use cases?

That is, if we are going to make the system less robust, shouldn't we
show that it has real benefit?

And yes, I consider user space passing in a system call of 0x401 and
having it work the same as 0x1 as being less robust.

-- Steve
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