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

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Date:	Wed, 7 May 2014 13:14:22 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Andy Lutomirski <luto@...capital.net>
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


* 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.

At minimum it can be a "look we want this speedup in hardware" 
testcase to CPU designers.

Thanks,

	Ingo
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