[RFC 5/8] powerpc/slb: Add documentation to runtime patching of SLB encoding
mpe at ellerman.id.au
Wed Jul 22 15:51:03 AEST 2015
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);
* 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
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