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

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Date: Fri, 16 Feb 2024 13:05:20 +0100
From: Helge Deller <deller@....de>
To: Charlie Jenkins <charlie@...osinc.com>, Guenter Roeck <linux@...ck-us.net>
Cc: David Laight <David.Laight@...lab.com>,
 Palmer Dabbelt <palmer@...belt.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Parisc List <linux-parisc@...r.kernel.org>, Al Viro
 <viro@...iv.linux.org.uk>, 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 2/16/24 07:13, Charlie Jenkins wrote:
> On Thu, Feb 15, 2024 at 10:09:42PM -0800, Guenter Roeck wrote:
>> On 2/15/24 21:54, Helge Deller wrote:
>> [ ... ]
>>>
>>> Can you please give a pointer to this test code?
>>> I'm happy to try it on real hardware.
>>>
>> You should also see the problem if you use v7 of Charlie's checksum
>> unit test fixes.
>>
>> I submitted the qemu fix (or at least what I think the fix should be)
>> a couple of minutes ago.
>>
>> https://patchwork.kernel.org/project/qemu-devel/patch/20240216053415.2163286-1-linux@roeck-us.net/
>>
>>>> It is quite easy to show that carry is always set after executing ldd
>>>> on an unaligned address. That is also why I know for sure that the
>>>> problem is not seen with ldw on unaligned addresses.
>>> Interesting.
>>
>> Ultimately it wasn't surprising, with the unusual carry bit
>> implementation on hppa. The upper 8 carry bits were not masked
>> correctly when returning from a trap or interrupt.
>
> Tangential question, but why does Linux need to save and restore the PSW
> if that is already handled by the hardware? I am missing something.

e.g. for task switching.

Helge