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

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

> > 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.

Yes, so the answer is that we 'flip' about as often as the current
code emulates (e.g., about 2.5 million flips/emulations to boot a Red
Hat system).

The performance is very bad, but the flipping code is both simpler and
more robust than emulation so I will go with the new technique. But I
will still print a warning message from Linux to tell the user to
remove /lib/tls.

 -- Keir


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