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Re: [Xen-devel] [patch 17/44] Add nosegneg capability to the vsyscall pa

To: Jeremy Fitzhardinge <jeremy@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] [patch 17/44] Add nosegneg capability to the vsyscall page notes
From: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
Date: Tue, 17 Jul 2007 10:45:55 +1000
Cc: Zachary Amsden <zach@xxxxxxxxxx>, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>, Jeremy Fitzhardinge <jeremy@xxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Andi Kleen <ak@xxxxxxx>, lkml <linux-kernel@xxxxxxxxxxxxxxx>, Chris Wright <chrisw@xxxxxxxxxxxx>, Ian Pratt <ian.pratt@xxxxxxxxxxxxx>, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>, Ulrich Drepper <drepper@xxxxxxxxxx>, Roland McGrath <roland@xxxxxxxxxx>, Christian Limpach <Christian.Limpach@xxxxxxxxxxxx>
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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.



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