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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 09.08.2019 15:42, Andrew Cooper wrote:
> 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.
Oh, indeed, and not just for this aspect.

> > --- 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.
Will do.

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

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