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

[Juergen Gross <jgross@suse.com>]

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On 05.12.23 14:31, Julien Grall wrote:
> 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?

This is the 9pfs protocol (see [1]).

> 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?

Correct.

> 
>> 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?

This should happen through the "__packed" attribute on the access macros. As
e.g. MIPS doesn't support unaligned accesses, but is working with those access
macros in the Linux kernel, I suspect the attribute is doing its job.

> 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.

Like in xen/common/lzo.c et al? Those are compiled with CONFIG_X86 only today,
but I guess other archs might need the decompressors in future, too.


Juergen

[1]: http://ericvh.github.io/9p-rfc/rfc9p2000.html

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