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 00:56:13 -0800
From: Guenter Roeck <linux@...ck-us.net>
To: Charlie Jenkins <charlie@...osinc.com>,
 John David Anglin <dave.anglin@...l.net>
Cc: 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 2/14/24 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