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 07:22:32 -0800
From: Guenter Roeck <linux@...ck-us.net>
To: John David Anglin <dave.anglin@...l.net>,
 Charlie Jenkins <charlie@...osinc.com>
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 19:00, 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.
> 
> However, it appears the tophys macro might clobber the carry bits before they
> are saved in intr_save.
> 

Actually, I did some logging and found that the correct carry bits do find their way
into the IPSW register in the Linux kernel. Only when userspace is back after handling
the unaligned LDD, the carry flag is always set to 1, no matter how it was set prior
to the unaligned access.

Guenter