Re: [PATCH 1/2] xen: make include/xen/unaligned.h usable on all architectures

From: Julien Grall <julien@xen.org>
To: Juergen Gross <jgross@suse.com>, xen-devel@lists.xenproject.org
Cc: Andrew Cooper <andrew.cooper3@citrix.com>,
	George Dunlap <george.dunlap@citrix.com>,
	Jan Beulich <jbeulich@suse.com>,
	Stefano Stabellini <sstabellini@kernel.org>, Wei Liu <wl@xen.org>,
	Arnd Bergmann <arnd@arndb.de>
Subject: Re: [PATCH 1/2] xen: make include/xen/unaligned.h usable on all architectures
Date: Tue, 5 Dec 2023 13:31:10 +0000	[thread overview]
Message-ID: <96ed8aa3-f92f-4b33-a846-549cfda14548@xen.org> (raw)
In-Reply-To: <e8df45d3-1d7d-432b-b0ca-7532d4b35eae@suse.com>

Hi Juergen,

On 05/12/2023 12:39, Juergen Gross wrote:
> On 05.12.23 12:53, Julien Grall wrote:
>> Hi Juergen,
>>
>> On 05/12/2023 10:07, Juergen Gross wrote:
>>> Instead of defining get_unaligned() and put_unaligned() in a way that
>>> is only supporting architectures allowing unaligned accesses, use the
>>> same approach as the Linux kernel and let the compiler do the
>>> decision how to generate the code for probably unaligned data accesses.
>>>
>>> Update include/xen/unaligned.h from include/asm-generic/unaligned.h of
>>> the Linux kernel.
>>>
>>> The generated code has been checked to be the same on x86.
>>>
>>> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
>>> Origin: 
>>> git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git 
>>> 803f4e1eab7a
>>> Signed-off-by: Juergen Gross <jgross@suse.com>
>>
>> Can you outline your end goal? At least on arm32, I believe this will 
>> result to abort because event if the architecture support unaligned 
>> access, we are preventing them on Arm32.
> 
> I need something like that in Xen tools for supporting packed data accesses
> on the 9pfs ring page, so I looked into the hypervisor for related support.

Did we really introduce an ABI requiring unaligned access??? Or is this 
something you are coming up with?

Anyway, IIRC Linux allows unaligned access. So the problem I am 
describing is only for the hypervisor. Although, I would like to point 
out that unaligned access has no atomicity guarantee. I assume this is 
not going to be a concern for you?

> I guess for arm32 using -mno-unaligned-access when building should avoid 
> any
> unaligned accesses?

I am not sure. This is implies the compiler will be able to infer that 
the access will be unaligned. Is this always the case?

Anyway, given you don't seem to have a use-case yet, I would simply to 
consider to surround the declaration with an a config which can be 
selected if unaligned access is supported.

Cheers,

-- 
Julien Grall