linux-kernel - Re: [PATCH 2/3] x86: Implement prctl PR_GET_TSC and PR_SET

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Date:	Sat, 12 Apr 2008 23:05:44 +0200 (CEST)
From:	Erik Bosman <ejbosman@...vu.nl>
To:	"H. Peter Anvin" <hpa@...or.com>
cc:	Arjan van de Ven <arjan@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Andrea Arcangeli <andrea@...share.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] x86: Implement prctl PR_GET_TSC and PR_SET_TSC



On Sat, 12 Apr 2008, H. Peter Anvin wrote:

> Arjan van de Ven wrote:
> >
> > Hi,
> >
> > why did you make this a configuration option? In general it's not
> > a good idea to make userspace visible ABI's (PR_* clearly is one of these)
> > a CONFIG option unless there's some HUGE space saving going on.
> > I don't see that here....
> >
> > So can you explain your rationale for making this a config option?
> >

The ABI itself is not a config option (see patch 1/3.)
If the x86 implementation patch isn't applied, prctl() will
return -EINVAL, just like most other PR_* options, which are
only implemented on specific architectures, take (PR_GET/PR_SET_ENDIAN)
as an example.

>
> I also saw no mention about performance impact, which need to be
> considered whenever *anything* is proposed to be inserted into a hot
> path.  It may be (heck, *should be*) that the performance impact isn't
> measurable, but that needs to be positively established.
>

This is why I made the implementation part configurable, although I don't
think the overhead will be very high since it seems to me that the
__switch_to_xtra function was designed explicitly to keep unusual options
out of the hot path.


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