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: <0fa4d53a-6ce7-4f44-81b2-86282f0b9451@roeck-us.net>
Date: Thu, 15 Feb 2024 08:30:22 -0800
From: Guenter Roeck <linux@...ck-us.net>
To: Charlie Jenkins <charlie@...osinc.com>
Cc: John David Anglin <dave.anglin@...l.net>,
 David Laight <David.Laight@...lab.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>, 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 07:36, Charlie Jenkins wrote:
> On Thu, Feb 15, 2024 at 12:56:13AM -0800, Guenter Roeck wrote:
>> On 2/14/24 19:35, Charlie Jenkins wrote:
>>> On Wed, Feb 14, 2024 at 10:00:37PM -0500, John David Anglin wrote:
>>>> On 2024-02-14 8:58 p.m., Guenter Roeck wrote:
>>>>> Specifically: Yes, the carry/borrow bits should be restored. Question is
>>>>> if the Linux kernel's interrupt handler doesn't restore the carry bits
>>>>> or if the problem is on the qemu side.
>>>> The carry/borrow bits in the PSW should be saved and restored by the save_specials
>>>> and rest_specials macros.  They are defined in arch/parisc/include/asm/assembly.h.
>>>
>>> Why would they be needed to be restored in linux? The manual says "The
>>> PSW is set to the contents of the IPSW by the RETURN FROM INTERRUPTION
>>> instruction". This means that the PSW must be restored by the hardware.
>>>
>>> We can see the QEMU implementation in:
>>>
>>> rfi:
>>> https://github.com/qemu/qemu/blob/v8.2.1/target/hppa/sys_helper.c#L93
>>>
>>> handling interrupt:
>>> https://github.com/qemu/qemu/blob/v8.2.1/target/hppa/int_helper.c#L109
>>>
>>> However the implementation appears to be faulty. During an RFI, the PSW
>>> is always set to 0x804000e (regardless of what the PSW was before the
>>> interrupt).
>>>
>>
>> Not sure if I agree. The interrupt handler in Linux is the one which needs to set
>> IPSW. Looking into the code, I agree with Dave that the tophys macro seems to
>> clobber the carry bits before psw is saved, so they can not really be restored.
>> The only issue with that idea is that I can only reproduce the problem with
>> an interrupted ldd instruction but not, for example, with ldw. This is why it
>> would be really important to have someone with real hardware test this.
>>
>> Thanks,
>> Guenter
> 
> Yes, we definitely feedback from somebody with access to hardware, but I
> do not understand how "The PSW is set to the contents of the IPSW by the
> RETURN FROM INTERRUPTION" could be interpreted as anything except that
> the hardware is expected to over-write the contents of the PSW during
> the rfi.
> 

Sure, I absolutely agree. But that assumes that IPSW is set correctly
in the Linux interrupt handler. We do know that something odd happens
when an unaligned ldd is encountered. At least for my part I don't know
if the problem is in emulate_ldd() in the Linux kernel or in the ldd
implementation and trap handling in qemu. I do know (from my logs)
that qemu does see the correct PSW/IPSW values, because they do
show up correctly in the Linux kernel when running the qemu emulation.
Only it somehow gets lost when the Linux interrupt handler returns.

Thanks.
Guenter