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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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