[5/8] powerpc/slb: Add documentation to runtime patching of SLB encoding

[Michael Ellerman]

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 <khandual at 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