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

[Michael Ellerman]

On Tue, 2015-07-21 at 12:28 +0530, Anshuman Khandual wrote:
> From: "khandual at linux.vnet.ibm.com" <khandual at linux.vnet.ibm.com>
> 
> 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.
> 
> diff --git a/arch/powerpc/mm/slb.c b/arch/powerpc/mm/slb.c
> index dcba4c2..8083a9e 100644
> --- a/arch/powerpc/mm/slb.c
> +++ b/arch/powerpc/mm/slb.c
> @@ -278,7 +278,13 @@ 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;
> +	/*
> +	 * Currently this patches only "li" and "cmpldi"
> +	 * instructions with an immediate value. Here it
> +	 * just clears the existing immediate value from
> +	 * the instruction and inserts a new one there.
> +	 */
> +	unsigned int insn = (*insn_addr & 0xffff0000) | immed;
>  	patch_instruction(insn_addr, insn);
>  }


How about:

	/*
	 * 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 ori) and cmpldi both take a 16-bit immediate
	 * value, and it is situated in the same location in the instruction,
	 * ie. bits 0-15.
	 * To patch the value we read the existing instruction, clear the
	 * immediate value, and or in our new value, then write the instruction
	 * back.
	 */

cheers