riscv: add FPE code

[br]

Summary

add floating point extension (FPE) code needed for bhyve and vmm_switch()

[jrtc27]

sys/riscv/riscv/fpe.c

106

This doesn't capture the memory side-effects. You either need a : "memory" in there (a big hammer, though probably doesn't make much difference here) or an extra dummy operand "=m"(*(uint64_t (*)[32][2])fp_x).

[jrtc27]

sys/riscv/riscv/fpe.c

106

Though if doing the latter, probably nicer to do:

uint64_t fcsr, (*fp_x)[32][2];

fp_x = &regs->fp_x;

and then you don't need to cast in the inline assembly, just "=m"(*fp_x).

(Yes, the syntax for a pointer to an array is real, if extremely rarely used, and yes it should remind you of function pointers...)

[br]

• Add dummy operand "m" to tell compiler which memory we read/write via pointers.
• Ensure all operands are output in fpe_store() and input in fpe_restore().

[mhorne]

Thanks for this. One tweak required.

sys/riscv/riscv/fpe.c

177

191

[jrtc27]

• Ensure all operands are output in fpe_store() and input in fpe_restore().

This was wrong; the r constraint for fp_x is meant to be an input in fpe_store too, as the address is an input, not an output. D53441.