On Thu, Sep 22, 2016 at 08:15:00AM +0000, Ed Schouten wrote:
> Author: ed
> Date: Thu Sep 22 08:14:59 2016
> New Revision: 306162
> URL: https://svnweb.freebsd.org/changeset/base/306162
> Log:
>   Make it possible to safely use TPIDRURW from userspace.
>   On amd64, arm64 and i386, we have the possibility to switch between TLS
>   areas in userspace. The nice thing about this is that it makes it easier
>   to do light-weight threading, if we ever feel like doing that. On armv6,
>   let's go into the same direction by making it possible to safely use the
>   TPIDRURW register, which is intended for this purpose.

What do you mean about 'possibility to switch between TLS areas in
usermode' ? On x86, kernel switches %fs/%gs bases on the context
switches. There is, indeed, a way to turn off this functionality, but it
is somewhat unobvious, I have to admit.

E.g. on Ivy+, RDFSBASE/RDGSBASE work.  WRFSBASE/WRGSBASE are enabled,
but they are de-facto not functional, since next context switch overrides
the base, in default setup.
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