linux-kernel - Re: [Xen-devel] [patch 17/44] Add nosegneg capability to the vsyscall page notes

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Date:	Mon, 16 Jul 2007 18:04:54 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	Jeremy Fitzhardinge <jeremy@...source.com>,
	Linus Torvalds <torvalds@...ux-foundation.com>,
	Zachary Amsden <zach@...are.com>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	Andi Kleen <ak@...e.de>, lkml <linux-kernel@...r.kernel.org>,
	Chris Wright <chrisw@...s-sol.org>,
	Ian Pratt <ian.pratt@...source.com>,
	Andrew Morton <akpm@...ux-foundation.com>,
	Ulrich Drepper <drepper@...hat.com>,
	Roland McGrath <roland@...hat.com>,
	Christian Limpach <Christian.Limpach@...cam.ac.uk>
Subject: Re: [Xen-devel] [patch 17/44] Add nosegneg capability to the	vsyscall
 page notes

Rusty Russell wrote:
> On Mon, 2007-07-16 at 16:15 -0700, Jeremy Fitzhardinge wrote:
>   
>> plain text document attachment (xen-vsyscall-note.patch)
>> Add the "nosegneg" fake capabilty to the vsyscall page notes. This is
>> used by the runtime linker to select a glibc version which then
>> disables negative-offset accesses to the thread-local segment via
>> %gs. These accesses require emulation in Xen (because segments are
>> truncated to protect the hypervisor address space) and avoiding them
>> provides a measurable performance boost.
>>     
>
> Hmm, this is still unconditional?  Not that it causes any measurable
> slowdown when enabled, but ISTR discussing making this dynamic...

Played with it for a bit, but it's fairly fiddly.  Didn't seem like it
was worth the complexity.

    J

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