linux-kernel - Re: More breakage in native_rdtsc out of line in git-x86

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Message-ID: <20080109163017.GG17739@elte.hu>
Date:	Wed, 9 Jan 2008 17:30:18 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	tglx@...utronix.de, linux-kernel@...r.kernel.org,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: More breakage in native_rdtsc out of line in git-x86

* Andi Kleen <andi@...stfloor.org> wrote:

> On Wed, Jan 09, 2008 at 04:22:08PM +0100, Ingo Molnar wrote:
> > 
> > * Andi Kleen <andi@...stfloor.org> wrote:
> > 
> > > > nope, it's a 64-bit setup/dependency bug/problem: the vsyscall mappings 
> > > > are installed via an __initcall, and that's too late for early use. The 
> > > > combo patch below fixed the crash for me, does it work on your box too?
> > > 
> > > That gives
> > > 
> > > /home/lsrc/quilt/linux/arch/x86/kernel/setup_64.c: In function 'setup_arch':
> > > /home/lsrc/quilt/linux/arch/x86/kernel/setup_64.c:468: error: implicit declarati
> > > on of function 'map_vsyscall'
> > 
> > i guess you have an old repository.
> 
> I just applied the patch you sent out against yesterday's git-x86.

then you have a truly ancient x86.git repository ;-)

think of x86.git#mm as an open development tree. It's high-flux, based 
against Linux-bleeding-edge, it's frequently updated (daily, sometimes 
hourly), breakages are possible (and likely) and fixes and other 
feedback is more than welcome. And please feel free to complain about 
patches that are included. (like you did in the past) Also please try to 
post your patches as early as possible instead of in big chunks - last 
week's 75 patches patchbomb from you was (and still is) ... challenging 
;-)

> > since yesterday there's a full barrier around rdtsc.
>
> Great.

i have measured the impact of the barriers and it was in the noise 
level. Barriers are notoriously easy to get wrong (because almost 
nothing tells the programmer that they are wrong), that's why i did this 
barrier-safe rdtsc() [& friends]. We had so much trouble with RDTSC 
during the past 10 years of its existence that being a bit more 
conservative with it is the only really maintainable option.

	Ingo
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