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>
diff --git a/arch/powerpc/mm/slb.c b/arch/powerpc/mm/slb.c
index faf9f0c..8a32a2b 100644
--- a/arch/powerpc/mm/slb.c
+++ b/arch/powerpc/mm/slb.c
@@ -249,7 +249,21 @@
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);
}
