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

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

Yes, I think this would work! :-)

I'll play around with expands-down segments a bit and, if they work as
desribed, I'll add the flipping idea to Xen. I'm pretty sure it'll
save us quite a few emulations, particularly if the +ve offsets are
accessed only infrequently.

It'll be interesting to see if it's good enough to be able to remove
the warning about /lib/tls.

 Thanks,
 Keir


> 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 ?


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