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Date: Thu, 15 Feb 2024 22:09:42 -0800
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Subject: Re: [PATCH v8 2/2] lib: checksum: Use aligned accesses for
 ip_fast_csum and csum_ipv6_magic tests
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To: Helge Deller <deller@gmx.de>, Charlie Jenkins <charlie@rivosinc.com>
Cc: David Laight <David.Laight@aculab.com>,
 Palmer Dabbelt <palmer@dabbelt.com>,
 Andrew Morton <akpm@linux-foundation.org>,
 Parisc List <linux-parisc@vger.kernel.org>, Al Viro
 <viro@zeniv.linux.org.uk>, linux-kernel@vger.kernel.org
References: <20240214-fix_sparse_errors_checksum_tests-v8-0-36b60e673593@rivosinc.com>
 <20240214-fix_sparse_errors_checksum_tests-v8-2-36b60e673593@rivosinc.com>
 <2ec91b11-23c7-4beb-8cef-c68367c8f029@roeck-us.net> <Zc1pSi59aDOnqz++@ghost>
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From: Guenter Roeck <linux@roeck-us.net>
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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.

> 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