Re: [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 08:51, John David Anglin wrote:
On 2024-02-15 10:44 a.m., Guenter Roeck wrote:
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.

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


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)


static bool pc_is_kernel_fn(unsigned long pc, void *fn)
        return (unsigned long)dereference_kernel_function_descriptor(fn) == pc;
This looks wrong to me.  Function descriptors should always be 8-byte aligned.  I think this
routine should return false if fn isn't 8-byte aligned.

Below you state "Code entry points only need 4-byte alignment."

I think that contradicts each other. Also, the calling code is,
for example,
	pc_is_kernel_fn(pc, syscall_exit)

I fail to see how this can be consolidated if it is ok
that syscall_exit is 4-byte aligned but, at the same time,
must be 8-byte aligned to be considered to be a kernel function.




 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 ?


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
Code entry points only need 4-byte alignment.
        mtctl   %r0, %cr0               /* Needed for single stepping */
@@ -1702,6 +1703,7 @@ ENTRY_CFI(sys_rt_sigreturn_wrapper)
        LDREG   PT_GR28(%r1),%r28  /* reload original r28 for syscall_exit */

+       .align 8
        /* 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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