RISC-V: hwprobe: Document unaligned vector perf key

Document key for reporting the speed of unaligned vector accesses.
The descriptions are the same as the scalar equivalent values.

Signed-off-by: Jesse Taube <jesse@rivosinc.com>
Reviewed-by: Charlie Jenkins <charlie@rivosinc.com>
Link: https://lore.kernel.org/r/20241017-jesse_unaligned_vector-v10-6-5b33500160f8@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Documentation/arch/riscv/hwprobe.rst

@@ -274,3 +274,19 @@ The following keys are defined:
   represent the highest userspace virtual address usable.
 
 * :c:macro:`RISCV_HWPROBE_KEY_TIME_CSR_FREQ`: Frequency (in Hz) of `time CSR`.
+
+* :c:macro:`RISCV_HWPROBE_KEY_MISALIGNED_VECTOR_PERF`: An enum value describing the
+     performance of misaligned vector accesses on the selected set of processors.
+
+  * :c:macro:`RISCV_HWPROBE_MISALIGNED_VECTOR_UNKNOWN`: The performance of misaligned
+    vector accesses is unknown.
+
+  * :c:macro:`RISCV_HWPROBE_MISALIGNED_VECTOR_SLOW`: 32-bit misaligned accesses using vector
+    registers are slower than the equivalent quantity of byte accesses via vector registers.
+    Misaligned accesses may be supported directly in hardware, or trapped and emulated by software.
+
+  * :c:macro:`RISCV_HWPROBE_MISALIGNED_VECTOR_FAST`: 32-bit misaligned accesses using vector
+    registers are faster than the equivalent quantity of byte accesses via vector registers.
+
+  * :c:macro:`RISCV_HWPROBE_MISALIGNED_VECTOR_UNSUPPORTED`: Misaligned vector accesses are
+    not supported at all and will generate a misaligned address fault.