From 79d0be7407955a268bcee5a33e338e14fdc74bfa Mon Sep 17 00:00:00 2001 From: Anshuman Khandual Date: Wed, 29 Jul 2015 12:40:02 +0530 Subject: 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 [mpe: Update change log and mention the signedness of the immediate] Signed-off-by: Michael Ellerman 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 @@ 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); } -- cgit v0.10.2