powerpc/slb: Add documentation on runtime patching of SLB encoding

This patch adds some documentation to patch_slb_encoding() explaining
how it works.

Signed-off-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
[mpe: Update change log and mention the signedness of the immediate]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/slb.c

@@ -249,7 +249,21 @@ void switch_slb(struct task_struct *tsk, struct mm_struct *mm)
 static inline void patch_slb_encoding(unsigned int *insn_addr,
 				      unsigned int immed)
 {
-	int insn = (*insn_addr & 0xffff0000) | immed;
+
+	/*
+	 * This function patches either an li or a cmpldi instruction with
+	 * a new immediate value. This relies on the fact that both li
+	 * (which is actually addi) and cmpldi both take a 16-bit immediate
+	 * value, and it is situated in the same location in the instruction,
+	 * ie. bits 16-31 (Big endian bit order) or the lower 16 bits.
+	 * The signedness of the immediate operand differs between the two
+	 * instructions however this code is only ever patching a small value,
+	 * much less than 1 << 15, so we can get away with it.
+	 * To patch the value we read the existing instruction, clear the
+	 * immediate value, and or in our new value, then write the instruction
+	 * back.
+	 */
+	unsigned int insn = (*insn_addr & 0xffff0000) | immed;
 	patch_instruction(insn_addr, insn);
 }