thread-ready ABIs

[Roland McGrath]

> The time is near when we (well, I) well start a drastic move toward
> generally using thread registers.  Even in non-threaded code.

Can you elaborate on this a little?  Do you just mean removing the
conditionalization in __libc_tsd_{get,set} and always calling the
functions?  A platform without thread registers could always just have
implementations of __libc_internal_tsd_* in -lpthread and -lc.

> Oh, this now also concerns Hurd.  So, Roland, how far is using LDTs on
> Hurd/x86?

I think it might be broken now, but we cna fix it.  We certainly intend to
have the feature (and/or something else equivalent for this purpose) and I
don't have a problem committing to relying on that in the libc-2.3 timeframe.

(The LDT approach is kind of gratuitous overhead for just this use of
emulating a thread register.  I might instead implement a simpler plan of a
constant %gs value pointing to a single location per CPU that the kernel
updates with a thread-specific word at context-switch time.)