The test fails with 11 errors on UP kernel 2.4.27. This is probably incorrect as SuperSparc should be IEEE 754 compliant. But a problem is that it fails with 21 errors on SMP kernel 2.4.27. I suppose that a different behavior between UP and SMP version is a serious bug. I see it is done the same way in 2.6.10 series.
I dug into a problem and found that it comes from not copying the FPU state in do_fork function. Actually, the FPU is disabled in every new created task and enabled later when the FPU operation is done. So the glibc test must have failed, because it enables or disables FP exceptions than forks and wait for the FP exception signal. After the fork, the FPU in the new task is initialized to a default state so rounding and exception settings are lost.
I also observed that a system of handling FPU context reloading is different in SMP and UP version.
I did a little research on Internet and found that FPU context initialization and reloading happened to other architectures (MIPS, SH) too. Various arguments were given how it should be done.
How should it be done? Should copy_flags() during do_fork() preserve FPU context/registers for child task? Should SMP and UP context switching be handled differently?
I am eager to fix this, Krzysztof Helt
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