[Xen-devel] Re: NPTL/TLS "emulation" idea (fwd)

[Rik van Riel <riel@xxxxxxxxxx>]

Hummm ok, so the list doesn't allow posting by non-members ...

---------- Forwarded message ----------
Date: Tue, 19 Oct 2004 12:45:01 -0700
From: Roland McGrath <roland@xxxxxxxxxx>
To: Rik van Riel <riel@xxxxxxxxxx>
Cc: xen-devel@xxxxxxxxxxxxxxxxxxxxx, Jakub Jelinek <jakub@xxxxxxxxxx>
Subject: Re: NPTL/TLS "emulation" idea

> A few weeks ago Roland, Jakub and myself brainstormed
> about this problem.  One of the things that came up is
> that the positive (glibc private data) and -ve (TLS)
> data are not generally used at the same time.

Well, that's not really true.  Small positive offsets are used all the time
(every syscall, for example, and all of pthreads internals).  Negative
offsets are used for actual ELF TLS accesses (__thread variables), which
now include `errno' in the standard glibc build.  So depending on your code
one or the other might be most common, but you are unlikely ever to have a
program run that doesn't flip back and forth a fair bit.  I really don't
have any clue what the fault-segment-flip-resume overhead vs the
fault-emulate-resume overhead is.  You'd just have to test it out.

I am still brainstorming about this, but I will need to do some experiments
to figure out how some other funny ways of using segments actually work.



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