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Subject[regression] commit d298b03506d3 ("x86/fpu: Restore the masking out of reserved MXCSR bits")

I have a 32bit installation here that stopped working. Bisected it
to commit d298b03506d3 ("x86/fpu: Restore the masking out of reserved
MXCSR bits").

dhcpcd was the first thing I notice being affected on account of
network not coming up, and after trying to look at it with gdb also
gdb turned out to be broken.

strace of dhcpcd shows a SIGFPE getting delivered, after which it gets
stuck (seem to be sitting in poll but not responding to even ^C).
And gdb seems to be stuck in a perpetual SIGFPE loop and won't even
get to the prompt.

The crucial bit here seems to be that most of the software is built
with -mfpmath=sse. After rebuilding dhcpcd without that it started
to work on the broken kernel. Rebuilding gdb didn't help so I whatever
SSE usage is causing the issue is presumably happening in a library.
Had to do the rebuilds on a working kernel as well because otherwise
the build itself would die to a SIGFPE somewhere.

Tested the same disk on on both a 64bit capable Pentium D
and a 32bit only Pentium 4 just to rule out the specific CPU.
Busted on both.

Ville Syrjälä

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