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From: Phil Blundell <philb@gnu.org>
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Date: Sun, 11 Sep 2011 22:33:20 +0100
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On Sat, 2011-09-10 at 10:28 +0100, Phil Blundell wrote: 
> The other odd thing is that, from a quick look at the qemu source code,
> it does seem as though sys_futex (which is syscall 240 on i386 and
> syscall 202 on x86-64) ought to be supported if qemu was built with NPTL
> on.  However, in my x86-64 test I did see the "unsupported syscall 202"
> syscall although the binary still seemed to run fine.

This part turns out not (quite) to be so mysterious.  For some reason,
qemu's configure script doesn't set target_nptl=yes for i386 or x86-64,
though it does for the majority of other arches.  So it will indeed be
the case that sys_futex will show up as an "unsupported syscall" on
those two architectures.

I'm not quite sure why qemu would be deciding not to do futexes on those
two architectures specifically.  I guess there must be some
functionality missing from the x86 emulation backend though I am not
sure exactly what that would be or how hard it would be to add.

p.