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

Rusty Russell
2007-02-14T05:39:44Z

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.