Re: [Xen-devel] [PATCH v2 1/2] xen/link: Introduce .bss.percpu.page_aligned

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Fri, Jul 26, 2019 at 09:32:21PM +0100, Andrew Cooper wrote:
> Future changes are going to need to page align some percpu data.
> 
> This means that the percpu area needs suitably aligning in the BSS so CPU0 has
> correctly aligned data.  Shuffle the exact link order of items within the BSS
> to give .bss.percpu.page_aligned appropriate alignment.
> 
> In addition, we need to be able to specify an alignment attribute to
> __DEFINE_PER_CPU().  Rework it so the caller passes in all attributes, and
> adjust DEFINE_PER_CPU{,_READ_MOSTLY}() to match.  This has the added bonus
> that it is now possible to grep for .bss.percpu and find all the users.
> 
> Finally, introduce DEFINE_PER_CPU_PAGE_ALIGNED() which uses both section and
> alignment attributes.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

> diff --git a/xen/include/asm-x86/percpu.h b/xen/include/asm-x86/percpu.h
> index ff34dc7897..5b6cef04c4 100644
> --- a/xen/include/asm-x86/percpu.h
> +++ b/xen/include/asm-x86/percpu.h
> @@ -7,10 +7,8 @@ extern unsigned long __per_cpu_offset[NR_CPUS];
>  void percpu_init_areas(void);
>  #endif
>  
> -/* Separate out the type, so (int[3], foo) works. */
> -#define __DEFINE_PER_CPU(type, name, suffix)                    \
> -    __section(".bss.percpu" #suffix)                            \
> -    __typeof__(type) per_cpu_##name
> +#define __DEFINE_PER_CPU(attr, type, name) \
> +    attr __typeof__(type) per_cpu_ ## name
>  
>  /* var is in discarded region: offset to particular copy we want */
>  #define per_cpu(var, cpu)  \
> diff --git a/xen/include/xen/percpu.h b/xen/include/xen/percpu.h
> index aeec5c19d6..71a31cc361 100644
> --- a/xen/include/xen/percpu.h
> +++ b/xen/include/xen/percpu.h
> @@ -9,9 +9,15 @@
>   * 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) \
> +    __DEFINE_PER_CPU(__section(".bss.percpu.page_aligned") \
> +                     __aligned(PAGE_SIZE), type, _ ## name)
> +
>  #define DEFINE_PER_CPU_READ_MOSTLY(type, name) \
> -     __DEFINE_PER_CPU(type, _##name, .read_mostly)
> +    __DEFINE_PER_CPU(__section(".bss.percpu.read_mostly"), type, _ ## name)

AFAICT also adding a '_' here will result in variable names with
per_cpu__foo, which is inline with the previous behaviour, but I'm not
sure of the point of the double underscore.

Thanks, Roger.

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