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 09:06:14 -0800
From: Guenter Roeck <linux@...ck-us.net>
To: John David Anglin <dave.anglin@...l.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 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.
>>>
>>> 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.

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.

Guenter

>> }
>>
>> 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
>