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Re: [Xen-devel] [PATCH v4 1/2] xen/link: Introduce .bss.percpu.page_aligned


  • To: Jan Beulich <jbeulich@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Fri, 9 Aug 2019 14:42:12 +0100
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  • Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Konrad Wilk <konrad.wilk@xxxxxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Tim Deegan <tim@xxxxxxx>, Julien Grall <julien.grall@xxxxxxx>, Ian Jackson <ian.jackson@xxxxxxxxxx>
  • Delivery-date: Fri, 09 Aug 2019 13:42:20 +0000
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  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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On 09/08/2019 13:32, Jan Beulich wrote:
> Insert explicit alignment such that the result is safe even with objects
> shorter than a page in length.  The POINTER_ALIGN for __bss_end is to
> cover
> the lack of SMP_CACHE_BYTES alignment, as the loops which zero the BSS
> use
> pointer-sized stores on all architectures.

...

> v4:
>  * Drop stray trailing ALIGN(). Make DEFINE_PER_CPU_PAGE_ALIGNED() verify
>    the alignment rather than specifying it.

My feelings about the stray-ness of ALIGN() notwithstanding, the commit
message now wrong and needs correcting.

> --- a/xen/include/xen/percpu.h
> +++ b/xen/include/xen/percpu.h
> @@ -9,9 +9,17 @@
>   * The _##name concatenation is being used here to prevent 'name'
> from getting
>   * macro expanded, while still allowing a per-architecture symbol
> name prefix.
>   */
> -#define DEFINE_PER_CPU(type, name) __DEFINE_PER_CPU(type, _##name, )
> +#define DEFINE_PER_CPU(type, name) \
> +    __DEFINE_PER_CPU(__section(".bss.percpu"), type, _ ## name)
> +
> +#define DEFINE_PER_CPU_PAGE_ALIGNED(type, name) \
> +    typedef char name ## _chk_t[BUILD_BUG_ON_ZERO(__alignof(type) & \
> +                                                  (PAGE_SIZE - 1))]; \
> +    __DEFINE_PER_CPU(__section(".bss.percpu.page_aligned"), \
> +                     type, _ ## name)

I think this would be easier to read as:

#define DEFINE_PER_CPU_PAGE_ALIGNED(type, name)                \
    typedef char name ## _chk_t[                               \
        BUILD_BUG_ON_ZERO(__alignof(type) & (PAGE_SIZE - 1))]; \
    __DEFINE_PER_CPU(__section(".bss.percpu.page_aligned"),    \
                     type, _ ## name)


By not breaking important bit of logic across a newline.

Preferably with this changed, but definitely with the commit message
fixed, Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

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