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

[Zachary Amsden <zach@xxxxxxxxxx>]

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.
>
> Signed-off-by: Ian Pratt <ian.pratt@xxxxxxxxxxxxx>
> Signed-off-by: Christian Limpach <Christian.Limpach@xxxxxxxxxxxx>
> Signed-off-by: Chris Wright <chrisw@xxxxxxxxxxxx>
>   
Acked-by: Zachary Amsden <zach@xxxxxxxxxx>

We would like to see this by dynamic, but that is much more difficult to 
achieve, and seeing your recent linker issues, I don't think this should 
gate merging this code.  The performance loss for us I believe to be 
negligible, and the fix is quite a bit more complicated than something 
achievable in the .21 timeframe.
Zach

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