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    SubjectRe: [PATCH 5/8] arm64: alternative: Add support for patching adrp instructions
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    On 22/08/16 12:19, Will Deacon wrote:
    > On Thu, Aug 18, 2016 at 02:10:29PM +0100, Suzuki K Poulose wrote:
    >> adrp uses PC-relative address offset to a page (of 4K size) of
    >> a symbol. If it appears in an alternative code patched in, we
    >> should adjust the offset to reflect the address where it will
    >> be run from. This patch adds support for fixing the offset
    >> for adrp instructions.
    >>
    >> Cc: Will Deacon <will.deacon@arm.com>
    >> Cc: Marc Zyngier <marc.zyngier@arm.com>
    >> Cc: Andre Przywara <andre.przywara@arm.com>
    >> Cc: Mark Rutland <mark.rutland@arm.com>
    >> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
    >> ---
    >> arch/arm64/kernel/alternative.c | 13 +++++++++++++
    >> 1 file changed, 13 insertions(+)
    >>
    >> diff --git a/arch/arm64/kernel/alternative.c b/arch/arm64/kernel/alternative.c
    >> index d2ee1b2..71c6962 100644
    >> --- a/arch/arm64/kernel/alternative.c
    >> +++ b/arch/arm64/kernel/alternative.c
    >> @@ -80,6 +80,19 @@ static u32 get_alt_insn(struct alt_instr *alt, u32 *insnptr, u32 *altinsnptr)
    >> offset = target - (unsigned long)insnptr;
    >> insn = aarch64_set_branch_offset(insn, offset);
    >> }
    >> + } else if (aarch64_insn_is_adrp(insn)) {
    >> + s32 orig_offset, new_offset;
    >> + unsigned long target;
    >> +
    >> + /*
    >> + * If we're replacing an adrp instruction, which uses PC-relative
    >> + * immediate addressing, adjust the offset to reflect the new
    >> + * PC. adrp operates on 4K aligned addresses.
    >> + */
    >> + orig_offset = aarch64_insn_adrp_get_offset(insn);
    >> + target = ((unsigned long)altinsnptr & ~0xfffUL) + orig_offset;
    >> + new_offset = target - ((unsigned long)insnptr & ~0xfffUL);
    >
    > The masking with ~0xfffUL might be nicer if you write it as
    > align_down(ptr, SZ_4K);

    Right, that definitely looks better. Will change it.

    >
    >> + insn = aarch64_insn_adrp_set_offset(insn, new_offset);
    >> }
    >>
    >> return insn;
    >
    > I wonder if we shouldn't have a catch-all for any instructions performing
    > PC-relative operations here, because silent corruption of the instruction

    Correct, which is what happened initially when I didn't have the adrp handling ;-).

    > stream is pretty horrible. What other instructions are there? ADR, LDR
    > (literal), ... ?

    From a quick look, all the instructions under "Load register (literal)" :

    i.e,
    LDR (literal) for GPR/FP_SIMD/32bit/64bit
    LDRSW (literal)
    PRFM (literal)

    and Data processing instructions - immediate group with PC-relative addressing:

    ADR, ADRP

    I will add a check to catch the unsupported instructions in the alternative code.

    Thanks
    Suzuki

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