arch/riscv: Document mprv_{read,write}_* functions

Change-Id: Iaf0cb241f0eb4de241f0983c0b32dbcc28f96480
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-on: https://review.coreboot.org/21342
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/arch/riscv/include/vm.h

@@ -91,6 +91,17 @@ void flush_tlb(void);
 		);							\
 	}
 
+/*
+ * mprv_{read,write}_* - Modified privilege memory access functions.
+ *
+ * These inline functions perform a read or write memory operation with the
+ * mstatus.MPRV bit set. This causes the memory protections and translation of
+ * the previous mode (e.g. U-mode, if we're handling a trap from U-mode) to be
+ * applied.
+ *
+ * The user of these functions must make sure to avoid trap loops through
+ * unaligned memory accesses.
+ */
 DEFINE_MPRV_READ(mprv_read_u8, uint8_t, lbu)
 DEFINE_MPRV_READ(mprv_read_u16, uint16_t, lhu)
 DEFINE_MPRV_READ(mprv_read_u32, uint32_t, lwu)