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    SubjectRe: [PATCH v8 2/2] lib: checksum: Use aligned accesses for ip_fast_csum and csum_ipv6_magic tests
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    On 2/15/24 02:27, David Laight wrote:
    > ...
    >> It would be worthwhile tracking this down since there are
    >> lots of unaligned data accesses (8-byte accesses on 4-byte aligned addresses)
    >> when running the kernel in 64-bit mode.
    >
    > Hmmm....
    > For performance reasons you really don't want any of them.
    > The misaligned 64bit fields need an __attribute((aligned(4)) marker.
    >
    > If the checksum code can do them it really needs to detect
    > and handle the misalignment.
    >
    > The misaligned trap handler probably ought to contain a
    > warn_on_once() to dump stack on the first such error.
    > They can then be fixed one at a time.
    >

    Unaligned LDD at unwind_once+0x4a8/0x5e0

    Decoded:

    Unaligned LDD at unwind_once (arch/parisc/kernel/unwind.c:212 arch/parisc/kernel/unwind.c:243 arch/parisc/kernel/unwind.c:371 arch/parisc/kernel/unwind.c:445)

    Source:

    static bool pc_is_kernel_fn(unsigned long pc, void *fn)
    {
    return (unsigned long)dereference_kernel_function_descriptor(fn) == pc;
    }

    Disassembled:

    c38: 50 3c 00 00 ldd 0(r1),ret0
    c3c: 08 1b 02 44 copy dp,r4
    c40: 0f 80 10 da ldd 0(ret0),r26 <--
    c44: 37 dd 3f a1 ldo -30(sp),ret1
    c48: 51 02 00 20 ldd 10(r8),rp
    c4c: e8 40 f0 00 bve,l (rp),rp
    c50: 51 1b 00 30 ldd 18(r8),dp

    Hmm, interesting. See below for a possible fix. Should I submit a patch ?

    Thanks,
    Guenter

    ---
    diff --git a/arch/parisc/kernel/entry.S b/arch/parisc/kernel/entry.S
    index ab23e61a6f01..1d2aab619466 100644
    --- a/arch/parisc/kernel/entry.S
    +++ b/arch/parisc/kernel/entry.S
    @@ -772,6 +772,7 @@ ENTRY_CFI(_switch_to)
    bv %r0(%r2)
    mtctl %r25,%cr30

    + .align 8
    ENTRY(_switch_to_ret)
    mtctl %r0, %cr0 /* Needed for single stepping */
    callee_rest
    @@ -1702,6 +1703,7 @@ ENTRY_CFI(sys_rt_sigreturn_wrapper)
    LDREG PT_GR28(%r1),%r28 /* reload original r28 for syscall_exit */
    ENDPROC_CFI(sys_rt_sigreturn_wrapper)

    + .align 8
    ENTRY(syscall_exit)
    /* NOTE: Not all syscalls exit this way. rt_sigreturn will exit
    * via syscall_exit_rfi if the signal was received while the process

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