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 01:13:55 -0500
From: Charlie Jenkins <charlie@...osinc.com>
To: Guenter Roeck <linux@...ck-us.net>
Cc: Helge Deller <deller@....de>, 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 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.

- Charlie

> 
> > In general I think it's quite important to differentiate between
> > running on qemu or running on physical hardware.
> 
> I know, that makes testing always tricky (not just with this
> architecture) because it is often not obvious if the problem
> is a problem in the tested code or a problem in the emulation.
> 
> > Qemu just recently got 64-bit support, and it's not yet behaving
> > like real hardware. One thing I noticed is, that read hardware
> > does not seem to jump into the exception handler twice, while
> > qemu does. So, if you run into an exception (e.g. unaligned ldd)
> > then if a second exception happens in the fault handler (e.g. second
> > unaligned ldd to resolve wrongly-coded code lookup), you will
> > get different behaviour between hardware and emulation.
> 
> Hmm, interesting. Makes me wonder how the real hardware handles such
> double traps.
> 
> > This is also the reason why qemu still fails to emulate newer
> > 64-bit Linux kernels which uses kernel modules.
> > 
> I don't use modules in my testing, so I'll leave that alone for
> anther day.
> 
> Cheers,
> Guenter
>