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xen-devel
[Xen-devel] Re: [patch 13/21] Xen-paravirt: Add nosegneg capability to t
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To: |
Jeremy Fitzhardinge <jeremy@xxxxxxxx> |
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Subject: |
[Xen-devel] Re: [patch 13/21] Xen-paravirt: Add nosegneg capability to the vsyscall page notes |
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From: |
Zachary Amsden <zach@xxxxxxxxxx> |
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Date: |
Tue, 13 Feb 2007 14:45:43 -0800 |
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Cc: |
Andrew Morton <akpm@xxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx, Ian Pratt <ian.pratt@xxxxxxxxxxxxx>, virtualization@xxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, Chris Wright <chrisw@xxxxxxxxxxxx>, Andi Kleen <ak@xxxxxx>, Christian Limpach <Christian.Limpach@xxxxxxxxxxxx> |
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Tue, 13 Feb 2007 14:45:05 -0800 |
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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.
Zach
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