RISC-V: add helper function to read the vector VLEN

VLEN describes the length of each vector register and some instructions
need specific minimal VLENs to work correctly.

The vector code already includes a variable riscv_v_vsize that contains
the value of "32 vector registers with vlenb length" that gets filled
during boot. vlenb is the value contained in the CSR_VLENB register and
the value represents "VLEN / 8".

So add riscv_vector_vlen() to return the actual VLEN value for in-kernel
users when they need to check the available VLEN.
Signed-off-by: Heiko Stuebner <heiko.stuebner@vrull.eu>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jerry Shih <jerry.shih@sifive.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20240122002024.27477-2-ebiggers@kernel.orgSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/include/asm/vector.h

@@ -284,4 +284,15 @@ static inline bool riscv_v_vstate_ctrl_user_allowed(void) { return false; }
 
 #endif /* CONFIG_RISCV_ISA_V */
 
+/*
+ * Return the implementation's vlen value.
+ *
+ * riscv_v_vsize contains the value of "32 vector registers with vlenb length"
+ * so rebuild the vlen value in bits from it.
+ */
+static inline int riscv_vector_vlen(void)
+{
+	return riscv_v_vsize / 32 * 8;
+}
+
 #endif /* ! __ASM_RISCV_VECTOR_H */