linux-kernel - Re: [PATCH v5 9/9] x86-64: Add CONFIG_UNSAFE

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Date:	Mon, 6 Jun 2011 23:54:21 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Andrew Lutomirski <luto@....edu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	pageexec@...email.hu, Andi Kleen <andi@...stfloor.org>,
	x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, Jesper Juhl <jj@...osbits.net>,
	Borislav Petkov <bp@...en8.de>,
	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>,
	Brian Gerst <brgerst@...il.com>,
	Louis Rilling <Louis.Rilling@...labs.com>,
	Valdis.Kletnieks@...edu
Subject: Re: [PATCH v5 9/9] x86-64: Add CONFIG_UNSAFE_VSYSCALLS to
 feature-removal-schedule


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

> > don't even bother making it a config option for now. If we can 
> > replace the vsyscall page with a page fault or int3 or whatever, 
> > and it's only used for the 'time()' system call, just do it.
> 
> Really?
> 
> I won't personally complain about the 200+ ns hit, but I'm sure 
> someone will cc: me on a regression report if there's no option.

Well, for 0.2 usecs to show up one would have to do like a million of 
them per second.

Why an application would want to query the *same value* from the 
kernel a million times per second is beyond me. vgettimeofday()? 
Sure. But vtime()?

Thanks,

	Ingo
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