Re: [Xen-devel] [patch 17/44] Add nosegneg capability to the vsyscall page notes

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

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.

    J


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