Xen devel - Dec 2004 - Question about /lib/tls

As I understand it - emulating the standard /lib/tls behaviour
efficiently and properly with Xen is tricky. However, a change in libc
could make it easier to implement or something? And this change could
work with Xen and outside Xen?

Now, it might be a while before such a change could be gotten into
upstream glibc, and even longer before it would trickle down to
distributions.

But, my question is that does the change break binary compatibility? 
If I were to recompile glibc myself, could I add a patch that would
make /lib/tls work for Xen and outside Xen? Could I use that glibc
without recompiling all my applications?

And ofcourse - was this change in libc just discussed as a possible
alternative, or does it already exist somewhere? If it doesn''t, would
it be hard to do? Would Xen need to be modified to take advantage of
this new support?

TIA,
-- Naked



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> As I understand it - emulating the standard /lib/tls 
> behaviour efficiently and properly with Xen is tricky. 
> However, a change in libc could make it easier to implement 
> or something? And this change could work with Xen and outside Xen?
Yes, with a few modifications to glibc we''d have a library that worked
fine on Xen and also on native Linux. I seriously doubt there''d be any
measureable performance loss.

The other problem is that the very latest versions of gcc will also
generate problematic code if the program uses the new thread local
variable attribute. Fortunately, almost nothing does right now. There''s
already a flag to cause gcc to generate code that doesn''t generate -ve
segment offsets, but its not the default. It would be great if it was...

We just haven''t had time to look into modifying libc, but it would be
great if someone could. I''d probably modify Xen''s seg fixup
code to
print out EIPs to collect a hit list of the most frequently executed
problematic cases, then go and fix those in the source: rather than
doing a -ve segment offset, access gs:0 to read out the start of the
thread control block, then access tcb_start-offset. The only slight
problem is that a register needs to be used to hold the tcb_start value,
so there might need to be a bit of juggling with the stack if there
isn''t an obvious unused register.  

Ian


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