On Wed, 2015-29-07 at 07:10:02 UTC, Anshuman Khandual wrote:
> This patch adds some documentation to 'patch_slb_encoding' function
> explaining about how it clears the existing immediate value in the
> given instruction and inserts a new one there.
>
> Signed-off-by: Anshuman Khandual <khand...@linux.vnet.ibm.com>
> ---
> arch/powerpc/mm/slb.c | 13 ++++++++++++-
> 1 file changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/mm/slb.c b/arch/powerpc/mm/slb.c
> index c87d5de..1962357 100644
> --- a/arch/powerpc/mm/slb.c
> +++ b/arch/powerpc/mm/slb.c
> @@ -279,7 +279,18 @@ 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.
> + * 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);
> }
As Segher pointed out the signedness of the immediate value differs between the
instructions, I added:
+ * 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.
cheers
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