linux-kernel - Re: [patch 13/21] Xen-paravirt: Add nosegneg capability to the vsyscall page notes

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Message-ID: <45D549ED.4000206@vmware.com>
Date:	Thu, 15 Feb 2007 22:06:37 -0800
From:	Zachary Amsden <zach@...are.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Andi Kleen <ak@....de>, Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, virtualization@...ts.osdl.org,
	xen-devel@...ts.xensource.com, Chris Wright <chrisw@...s-sol.org>,
	Ian Pratt <ian.pratt@...source.com>,
	Christian Limpach <Christian.Limpach@...cam.ac.uk>
Subject: Re: [patch 13/21] Xen-paravirt: Add nosegneg capability to the vsyscall
 page notes

Jeremy Fitzhardinge wrote:
> 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.
>
> Signed-off-by: Ian Pratt <ian.pratt@...source.com>
> Signed-off-by: Christian Limpach <Christian.Limpach@...cam.ac.uk>
> Signed-off-by: Chris Wright <chrisw@...s-sol.org>
>   

Acked-by: Zachary Amsden <zach@...are.com>

We would like to see this by dynamic, but that is much more difficult to 
achieve, and seeing your recent linker issues, I don't think this should 
gate merging this code.  The performance loss for us I believe to be 
negligible, and the fix is quite a bit more complicated than something 
achievable in the .21 timeframe.

Zach
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