[Xen-devel] NPTL/TLS "emulation" idea

[Rik van Riel <riel@xxxxxxxxxx>]

Hi,

Currently TLS wraparound segments are a problem in Xen,
since the kernel needs to be protected by keeping the
user space segments out of kernel space.

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.

This lead to one of us (I don't remember whom, sorry)
came up with the idea of "flipping the segment around"
whenever a segment violation happens on a segment that
should have been 4GB in size, and the fault happens on
an address between 0 and TASK_SIZE.

Having glanced briefly at some documentation on how
the segments works, it looks like it should be possible
to convert the positive (clipped) segment into an
Expands-down segment, allowing TLS to work transparently
without needing to emulate any instructions.

Provided the segment doesn't need to be "flipped" too
often, this could provide for reasonably high performance
TLS functionality, without the potential problems that
instruction rewriting brings with it.

What do you think ?

-- 
"Debugging is twice as hard as writing the code in the first place.
Therefore, if you write the code as cleverly as possible, you are,
by definition, not smart enough to debug it." - Brian W. Kernighan



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