linux-kernel - Re: [ABOMINATION] x86: Fast interrupt return to userspace

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Date:	Wed, 7 May 2014 08:27:09 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Gleb Natapov <gleb@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [ABOMINATION] x86: Fast interrupt return to userspace

On Wed, May 7, 2014 at 4:14 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Andy Lutomirski <luto@...capital.net> wrote:
>
>> > Whatever. I got enough profile data to say that it seems to have
>> > cut 'iret' overhead by at least two thirds. So it may not *work*,
>> > but from a "hey look, some random numbers" standpoint it is worth
>> > playing with.
>>
>> :)
>>
>> Is there actual interest in turning something like this into a real
>> patch?  It would almost certainly have to default off and no one
>> sane would ever use it except for special-purpose machines.
>
> The macro speedup looks rather impressive, and we've done ugly things
> for far smaller speedups.
>
> But I don't think it should be a 'special mode'. It either is made to
> work unconditionally and can be a prime speedup to be proud of in a
> politely disgusted fashion, or we don't want the complexity (and
> future bitrot) of some special switch.

The problem is that we'll break anything that expects to be able to
use more than the specified 128-byte redzone.  Doing that is currently
safe as long as no signals are delivered.

I could imagine something like Go blowing up badly.

We might be able to get away with automatically disabling this thing
if sigaltstack is enabled.

--Andy
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