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xen-devel
Re: [Xen-devel] Re: [patch 13/21] Xen-paravirt: Add nosegneg capability
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To: |
Zachary Amsden <zach@xxxxxxxxxx> |
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Subject: |
Re: [Xen-devel] Re: [patch 13/21] Xen-paravirt: Add nosegneg capability to the vsyscall page notes |
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From: |
Rusty Russell <rusty@xxxxxxxxxxxxxxx> |
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Date: |
Wed, 14 Feb 2007 16:38:46 +1100 |
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Cc: |
Andrew Morton <akpm@xxxxxxxx>, Jeremy Fitzhardinge <jeremy@xxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx, virtualization@xxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, Chris Wright <chrisw@xxxxxxxxxxxx>, Ian Pratt <ian.pratt@xxxxxxxxxxxxx>, Christian Limpach <Christian.Limpach@xxxxxxxxxxxx>, Andi Kleen <ak@xxxxxx> |
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Delivery-date: |
Tue, 13 Feb 2007 21:39:10 -0800 |
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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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