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From:   Arnd Bergmann <arnd@kernel.org>
Date:   Sat, 10 Apr 2021 21:10:47 +0200
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Subject: Re: Bogus struct page layout on 32-bit
To:     Matthew Wilcox <willy@infradead.org>
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On Sat, Apr 10, 2021 at 4:44 AM Matthew Wilcox <willy@infradead.org> wrote:
> +                       dma_addr_t dma_addr __packed;
>                 };
>                 struct {        /* slab, slob and slub */
>                         union {
>
> but I don't know if GCC is smart enough to realise that dma_addr is now
> on an 8 byte boundary and it can use a normal instruction to access it,
> or whether it'll do something daft like use byte loads to access it.
>
> We could also do:
>
> +                       dma_addr_t dma_addr __packed __aligned(sizeof(void *));
>
> and I see pahole, at least sees this correctly:
>
>                 struct {
>                         long unsigned int _page_pool_pad; /*     4     4 */
>                         dma_addr_t dma_addr __attribute__((__aligned__(4))); /*     8     8 */
>                 } __attribute__((__packed__)) __attribute__((__aligned__(4)));
>
> This presumably affects any 32-bit architecture with a 64-bit phys_addr_t
> / dma_addr_t.  Advice, please?

I've tried out what gcc would make of this:  https://godbolt.org/z/aTEbxxbG3

struct page {
    short a;
    struct {
        short b;
        long long c __attribute__((packed, aligned(2)));
    } __attribute__((packed));
} __attribute__((aligned(8)));

In this structure, 'c' is clearly aligned to eight bytes, and gcc does
realize that
it is safe to use the 'ldrd' instruction for 32-bit arm, which is forbidden on
struct members with less than 4 byte alignment. However, it also complains
that passing a pointer to 'c' into a function that expects a 'long long' is not
allowed because alignof(c) is only '2' here.

(I used 'short' here because I having a 64-bit member misaligned by four
bytes wouldn't make a difference to the instructions on Arm, or any other
32-bit architecture I can think of, regardless of the ABI requirements).

      Arnd