linux-kernel - Re: [PATCH v8 2/2] lib: checksum: Use aligned accesses for ip_fast_csum and csum_ipv6

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Date: Thu, 15 Feb 2024 11:51:39 -0500
From: John David Anglin <dave.anglin@...l.net>
To: Guenter Roeck <linux@...ck-us.net>, David Laight
 <David.Laight@...LAB.COM>, Charlie Jenkins <charlie@...osinc.com>,
 Palmer Dabbelt <palmer@...belt.com>,
 Andrew Morton <akpm@...ux-foundation.org>, Helge Deller <deller@....de>,
 "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
 Parisc List <linux-parisc@...r.kernel.org>, Al Viro <viro@...iv.linux.org.uk>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v8 2/2] lib: checksum: Use aligned accesses for
 ip_fast_csum and csum_ipv6_magic tests

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.
>>
>> 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;
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.
> }
>
> 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
Code entry points only need 4-byte alignment.
>  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
Ditto.
>  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

Dave

-- 
John David Anglin  dave.anglin@...l.net