linux-kernel - Re: [PATCH v2 08/10] x86-64: Emulate vsyscalls

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Message-ID: <20110530121532.GH22324@elte.hu>
Date:	Mon, 30 May 2011 14:15:32 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Andrew Lutomirski <luto@....edu>
Cc:	x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, Jesper Juhl <jj@...osbits.net>,
	Borislav Petkov <bp@...en8.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Jan Beulich <JBeulich@...ell.com>,
	richard -rw- weinberger <richard.weinberger@...il.com>,
	Mikael Pettersson <mikpe@...uu.se>
Subject: Re: [PATCH v2 08/10] x86-64: Emulate vsyscalls


* Andrew Lutomirski <luto@....edu> wrote:

> On Mon, May 30, 2011 at 6:59 AM, Ingo Molnar <mingo@...e.hu> wrote:
> >
> > * Andrew Lutomirski <luto@....edu> wrote:
> >
> >>
> >> I'd advocate waiting until glibc 2.14 comes out with this change:
> >>
> >> http://sourceware.org/git/?p=glibc.git;a=commit;h=a8509ca540427502bd955f35296ff7b727c7a8a1
> >>
> >> I want to add a warning (ratelimited to an extremely low rate) in v3
> >> whenever any of the vsyscalls get used telling users that their legacy
> >> code is suffering a performance impact, but it seems like bad form to
> >> tell people to build glibc from git to avoid a regression.
> >
> > But only statically built binaries would be impacted in practice,
> > right? The number of statically built binaries that heavily rely on
> > vsyscalls ought to be a very small set ...
> 
> With current glibc even dynamic binaries take the hit on time().

Indeed, you are right, i completely forgot that again :)

> With the emulation warning (coming in v3), I get (on a Fedora 
> 15-based VM):
> 
> [    0.635493] init[1] emulated legacy vsyscall time(); upgrade your
> code to avoid a performance hit. ip:ffffffffff600404 sp:7fff277fe9c8
> caller:3da3e9e27d in libc.so.6[3da3e00000+192000]

Ok, so we should leave the option enabled by default and distros can 
flip it as they upgrade/fix glibc, right?

Thanks,

	Ingo
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