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

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Message-Id: <1171431526.19842.108.camel@localhost.localdomain>
Date:	Wed, 14 Feb 2007 16:38:46 +1100
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Zachary Amsden <zach@...are.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Andrew Morton <akpm@...l.org>, xen-devel@...ts.xensource.com,
	Ian Pratt <ian.pratt@...source.com>,
	virtualization@...ts.osdl.org, linux-kernel@...r.kernel.org,
	Chris Wright <chrisw@...s-sol.org>, Andi Kleen <ak@....de>,
	Christian Limpach <Christian.Limpach@...cam.ac.uk>
Subject: Re: [Xen-devel] Re: [patch 13/21] Xen-paravirt: Add nosegneg
	capability to the vsyscall page notes

On Tue, 2007-02-13 at 14:45 -0800, Zachary Amsden wrote:
> 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.
> >   
> 
> I don't like this because now a kernel compiled with both CONFIG_XEN and 
> CONFIG_VMI has "nosegneg" turned on.  We don't actually require this for 
> performance or correctness, so it would be nice to be able to 
> dynamically turn it off instead of having it forced.

Ditto for lguest.

Rusty.


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