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Re: [Xen-devel] [patch 17/44] Add nosegneg capability to the vsyscall page notes
- To: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
- From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
- Date: Mon, 16 Jul 2007 18:04:54 -0700
- Cc: Zachary Amsden <zach@xxxxxxxxxx>, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Jeremy Fitzhardinge <jeremy@xxxxxxxxxxxxx>, lkml <linux-kernel@xxxxxxxxxxxxxxx>, Andi Kleen <ak@xxxxxxx>, 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>
- Delivery-date: Mon, 16 Jul 2007 18:03:34 -0700
- List-id: Xen developer discussion <xen-devel.lists.xensource.com>
Rusty Russell wrote:
> 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...
Played with it for a bit, but it's fairly fiddly. Didn't seem like it
was worth the complexity.
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