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


  • To: Julien Grall <julien@xxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
  • From: Juergen Gross <jgross@xxxxxxxx>
  • Date: Tue, 5 Dec 2023 14:41:11 +0100
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  • Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Arnd Bergmann <arnd@xxxxxxxx>
  • Delivery-date: Tue, 05 Dec 2023 13:41:16 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

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@xxxxxxxx>
Origin: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git 803f4e1eab7a
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>

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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