linux-kernel - Re: [PATCH RFC] x86: fix confusing name of /proc/cpuinfo "ht" flag

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Message-ID: <20091112081317.GB25345@elte.hu>
Date:	Thu, 12 Nov 2009 09:13:17 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Bartlomiej Zolnierkiewicz <bzolnier@...il.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Dave Jones <davej@...emonkey.org.uk>
Subject: Re: [PATCH RFC] x86: fix confusing name of /proc/cpuinfo "ht" flag


* H. Peter Anvin <hpa@...or.com> wrote:

> On 11/11/2009 10:59 PM, Ingo Molnar wrote:
> > 
> > * Bartlomiej Zolnierkiewicz <bzolnier@...il.com> wrote:
> > 
> >> "ht" flag indicates only ability to detect siblings not HT presence 
> >> itself.
> >>
> >> Inspired by:
> >> http://www.codemonkey.org.uk/2009/11/10/common-hyperthreading-misconception/
> > 
> > i think instead of changing 'ht' to 'ht_detect', it would be even 
> > more intuitive to only expose the 'ht' flag if the ht-detect 
> > capability is there _and_ if the number of siblings is 2 or more.
> > 
> > That way we dont change it - we just 'hide' the 'ht' string on the 
> > category of systems that can enumerate HT via the CPUID but dont 
> > actually have HyperThreading.
> 
> /proc/cpuinfo is a user-space visible ABI.  Changing it is bad 
> chicken.

Almost ... something is only an ABI if an actual application relies on 
the 'ht' flag being there on non-hyperthreading CPUs. We dont know 
whether there's any, but the likelyhood looks pretty low. Could park it 
a branch for some time to see whether anything trips up.

	Ingo
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