Re: [RFC 5/8] powerpc/slb: Add documentation to runtime patching of SLB encoding
From: Michael Ellerman <mpe@ellerman.id.au>
Date: 2015-07-22 05:51:04
On Tue, 2015-07-21 at 12:28 +0530, Anshuman Khandual wrote:
quoted hunk ↗ jump to hunk
From: "khandual@linux.vnet.ibm.com" <redacted> 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