linux-kernel - Re: [PATCH v2 00/13] Unify asm/unaligned.h around struct helper

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Date:   Thu, 16 Dec 2021 09:42:41 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Arnd Bergmann <arnd@...nel.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
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Subject: Re: [PATCH v2 00/13] Unify asm/unaligned.h around struct helper

On Thu, Dec 16, 2021 at 9:29 AM Ard Biesheuvel <ardb@...nel.org> wrote:
>
> CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS is used in many places to
> conditionally emit code that violates C alignment rules. E.g., there
> is this example in Documentation/core-api/unaligned-memory-access.rst:
>
> bool ether_addr_equal(const u8 *addr1, const u8 *addr2)
> {
> #ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
>   u32 fold = ((*(const u32 *)addr1) ^ (*(const u32 *)addr2)) |
>              ((*(const u16 *)(addr1 + 4)) ^ (*(const u16 *)(addr2 + 4)));
>   return fold == 0;
> #else

It probably works fine in practice - the one case we had was really
pretty special, and about the vectorizer doing odd things.

But I think we should strive to convert these to use
"get_unaligned()", since code generation is fine. It still often makes
sense to have that test for the config variable, simply because the
approach might be different if we know unaligned accesses are slow.

So I'll happily take patches that do obvious conversions to
get_unaligned() where they make sense, but I don't think we should
consider this some huge hard requirement.

                 Linus