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powerpc/slb: Add documentation on runtime patching of SLB encoding

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This patch adds some documentation to patch_slb_encoding() explaining
how it works.

Signed-off-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
[mpe: Update change log and mention the signedness of the immediate]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
This commit is contained in:
Anshuman Khandual 2015-07-29 12:40:02 +05:30 committed by Michael Ellerman
parent 2be682af48
commit 79d0be7407
1 changed files with 15 additions and 1 deletions
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16
arch/powerpc/mm/slb.c
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@ -249,7 +249,21 @@ 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.
* 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.
* 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);
}