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

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Date:	Tue, 17 Jul 2007 10:45:55 +1000
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Jeremy Fitzhardinge <jeremy@...source.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.com>,
	Zachary Amsden <zach@...are.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	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

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...

Rusty.


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