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Subject: Re: [Xen-devel] [patch 17/44] Add nosegneg capability to the
	vsyscall page notes
From: Rusty Russell <rusty@rustcorp.com.au>
To: Jeremy Fitzhardinge <jeremy@xensource.com>
Cc: Linus Torvalds <torvalds@linux-foundation.com>,
       Zachary Amsden <zach@vmware.com>, Jeremy Fitzhardinge <jeremy@goop.org>,
       Xen-devel <xen-devel@lists.xensource.com>, Andi Kleen <ak@suse.de>,
       lkml <linux-kernel@vger.kernel.org>, Chris Wright <chrisw@sous-sol.org>,
       Ian Pratt <ian.pratt@xensource.com>,
       Andrew Morton <akpm@linux-foundation.com>,
       Ulrich Drepper <drepper@redhat.com>, Roland McGrath <roland@redhat.com>,
       Christian Limpach <Christian.Limpach@cl.cam.ac.uk>
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Date: Tue, 17 Jul 2007 10:45:55 +1000
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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.