linux-kernel - Re: cmpxchg and x86 flags output

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Message-ID: <7d1c236f-80d1-2c58-be0e-6676769636b3@zytor.com>
Date:	Tue, 21 Jun 2016 17:09:03 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	David Howells <dhowells@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-arch <linux-arch@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: cmpxchg and x86 flags output

On 06/21/16 10:24, H. Peter Anvin wrote:
> On 06/21/16 02:06, David Howells wrote:
>>
>> However, there's probably not a great deal of difference to be had if the
>> inline asm codes the appropriate instruction in each case for something like
>> x86*.  The emitted code ought to look the same.  The second biggest win for
>> the intriniscs, I think, is the ability to ask the CMPXCHG instruction whether
>> it actually did anything rather than comparing the result.  I added two
>> variants, one that only returned the yes/no and one that passed back the value
>> as well as the yes/no.
>>
> 
> Right, and we want that either way.  The API change that you are
> proposing is definitely what we want; the specifics of the x86
> implementation is sort of orthogonal.
> 

So how do we make this move forward?

	-hpa