How to access the vector register in the header file in the spike simulator for custom instruction.

[TVS2004Chaitanya]

I am trying to write a custom instruction that does matrix multiplication. I pre loaded the rows of matrix B into the vector registers shown below:
asm volatile(
"vsetvli t0, %[col_b], e32, m1\n\t"
"vle32.v v0, (%[B1])\n\t"
"vle32.v v1, (%[B2])\n\t"
"vle32.v v2, (%[B3])\n\t"
"vle32.v v3, (%[B4])\n\t"
:
: [B1] "r" (&B_inp[0]), [B2] "r" (&B_inp[colB]), [B3] "r" (&B_inp[2colB]), [B4] "r" (&B_inp[3colB]), [col_b] "r" (vmax)
: "t0", "v0", "v1", "v2", "v3", "memory"
);

How do I access these registers in the .h header file for my custom instruction in riscv-isa-sim/riscv/insns.