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authorAnshuman Khandual <khandual@linux.vnet.ibm.com>2015-07-29 12:40:02 +0530
committerMichael Ellerman <mpe@ellerman.id.au>2015-08-12 15:04:52 +1000
commit79d0be7407955a268bcee5a33e338e14fdc74bfa (patch)
treee58dd02d7b1188ac5c2fd0c276a42275d730902e /arch/powerpc/mm
parent2be682af48e8236558da702fe67e178cfe7524a1 (diff)
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>
Diffstat (limited to 'arch/powerpc/mm')
-rw-r--r--arch/powerpc/mm/slb.c16
1 files changed, 15 insertions, 1 deletions
diff --git a/arch/powerpc/mm/slb.c b/arch/powerpc/mm/slb.c
index faf9f0c4e823..8a32a2be3c53 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);
}