[musl] Re: Thread pointer changes

On Fri, Jun 27, 2014 at 02:47:38PM -0700, Andy Lutomirski wrote:

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Due to that, any ARMv5 or earlier CPU will always have the kuser helper
page.  ARMv6 and later may or may not have the kuser helper page, but
there you're really building for a different ABI anyway (VFP-based) and
you also know that you have the thread registers.

so is it expected that the libc makes no attempt to provide
portable binary interface for armv5 and armv6?

The libc interface that applications make use should not have any
dependence on whether KUSER_HELPERS is enabled or disabled, the
presence of that page should be totally invisible to applications.

As of right now, an x86_64 libc can have good performance on any
recent kernel and will work correctly on any kernel.  From what you're
saying, it sounds impossible to implement such a thing on ARM without
fiddling with /proc.

Indeed. I wasn't even aware of the legacy vsyscall mess for x86_64,
and we're not using it in musl. Keeping it around seems like just a
matter of maintaining API/ABI compatibility with legacy versions of
glibc that are using it. On the other hand, the kuser helper page is
the ONLY way for a libc that's compatible with pre-v6 arm to get
working atomics and TLS.

Rich