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Subject2.4, 2.6, i686/athlon and LDT's
Hi all,

I've some questions about LDT usage WRT threads. I'm working currently
with 2.4 kernels (although will be moving to 2.6 relatively soon) and
have had a lot of difficulty finding decent resources to adequately
explain proper LDT usage (from really why they're used to the
complexities of actually using them).

Context here is process freezing/restoring and how to properly
save/restore LDT entries for a given process -- with 2.4, glibc will
call modify_ldt() whenever the pthreads library is linked in (regardless
of whether threads are actually used, and regardless of whether there
are multiple threads currently running) on i686/athlon platforms to set
up a thread-specific storage area. (I'm not looking to save/restore all
information for individual threads; that could get icky with multiple
threads running).

It appears that use of the LDT is to speed up context switching between
threads, although I haven't even found especially good references WRT
that. I have looked through the info in the IA Developers publications
and have whacked my head against Google quite a bit. However, every bit
of clarity I've found there has been offset by new confuzled bits.

I also have found it's possible to compile glibc not to use LDT's on
these platforms; does anyone have comparison information w/threads using
LDT's and without (performance, protection, etc)?

Also, 2.6 appears to have moved to a tss storage area attributed with
each thread in the thread_struct... I'll be digging into that soon
enough, but if anyone has quick information on comparing this to the 2.4
tss mechanism I'd appreciate it; mainly regarding whether LDT's are
involved and if there are any gotchas there, or if it's much simpler and
doesn't need anything special to use. Also if it doesn't use LDT's,
what's the performance impact in moving away from them?

I'm not looking for information on saving/restoring processes outside
the LDT-specific bits -- I'm set on pretty much everything else.

Thanks for your time,

-Tymm
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