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

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Message-ID: <e73bdc36-5fb1-4ea8-9f96-608eb1a9b6af@bell.net>
Date: Thu, 15 Feb 2024 12:25:03 -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 12:06 p.m., Guenter Roeck wrote:
> 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.
In the above call, syscall_exit is treated as a function pointer. It points to an 8-byte aligned
function descriptor.  The descriptor holds the actual address of the function.  It only needs
4-byte alignment.

Descriptors need 8-byte alignment for efficiency on 64-bit parisc. The pc and gp are accessed
using ldd instructions.

Dave

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