My reason for implementing lazy FPU saving is so that I can make use of
the FPU (or MMX) for memcpy and related activities in the kernel without
having to pay the price of saving state every single time the kernel is
entered/exited, even in interrupts. At least on my box (P233MMX), an FPU
memcpy is about 45% faster on a large block copy than rep movs when you
don't have to save the FPU's state. Even on the K7 it's worth using MMX
for zeropage/memcpy.
Please note that the first version of the patch I posted is buggy, which
I'll fix and post after I return home.
-ben
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