fpu - x87/SSE FPU bring-up + per-task state

Header: kernel/include/kernel/fpu.h Source: arch/i386/core/fpu.c

fpu_init() arms the floating-point unit at boot (prep for a hosted libc — printf("%f"), strtod, <math.h>):

• CR0.EM = 0 — use the real FPU, not emulation.
• CR0.MP = 1, CR0.TS = 0 — no lazy-switch trap.
• fninit — known clean state.
• CR4.OSFXSR when FXSAVE/FXRSTOR is available.

The clean default state captured at init seeds each new task’s save area.

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Per-task state

Each task carries a 512-byte, 16-byte-aligned FXSAVE area (FPU_STATE_SIZE = 512), saved/restored across context switches so concurrent FP-using tasks don’t clobber each other.

Function	Role
fpu_init()	Arm the unit at boot.
fpu_save(area_512)	fxsave the current FPU/SSE state.
fpu_restore(area_512)	fxrstor a saved state.
fpu_init_state(area_512)	Seed a fresh task’s area with the clean default.
fpu_imul(a,b) / fpu_isqrt(x)	Inline-x87 self-test helpers (fild/fmulp/fistp, fild/fsqrt/fistp) used by the test_fpu ktest to prove the unit is live.

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The scheduler calls fpu_save/fpu_restore on every context switch (see Internals §FPU/TLS). The test_fpu ktest validates the unit after fpu_init(). The toolchain/README.md “x87” note tracks the path toward hosted FP binaries.