linux-kernel - Re: [PATCH v5 8/9] x86-64: Emulate legacy vsyscalls

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Date:	Wed, 08 Jun 2011 08:32:18 +0200
From:	pageexec@...email.hu
To:	Andrew Lutomirski <luto@....edu>
CC:	Brian Gerst <brgerst@...il.com>, Ingo Molnar <mingo@...e.hu>,
	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>,
	Andi Kleen <andi@...stfloor.org>,
	Louis Rilling <Louis.Rilling@...labs.com>,
	Valdis.Kletnieks@...edu
Subject: Re: [PATCH v5 8/9] x86-64: Emulate legacy vsyscalls

On 7 Jun 2011 at 19:49, Andrew Lutomirski wrote:

> My patch (the version that's in tip/x86/vdso) more-or-less does that.
> It's something like six lines of code, including error handling.

my memory is about more complexity (else i would have done it that way
to avoid the current result of double kernel entries) but anyway it's
a moot point now that this code is headed for mainline.

> __bad_area_nosemaphore is not a fast path, and in fact I tried that
> for the very first version of this code that I wrote.  My recollection
> is that it's noticeably slower than int 0xcc because it has to go
> through the whole VMA lookup.

you can put this logic much earlier, even before taking any locks (but
then you'll have to fear the dreaded single cycle improvement brigade ;).

> If you want to submit a patch to switch from int 0xcc to
> __bad_area_nosemaphore, be my guest :)  It will have almost no effect
> on the complexity of the code, and, in fact, you'll probably get a net
> deletion of lines because you can remove all the crud in vmlinux.lds.S
> and some of the mapping code in vsyscall_64.c.

i'll forward port my changes when 3.0 comes out, we'll see how more or
less complex they become and can then decide if it's worth it.

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