Re: [PATCH 4/4] xen/x86: Correct mandatory and SMP barrier definitions

[Juergen Gross <jgross@suse.com>]

On 05/12/16 11:05, Andrew Cooper wrote:
> Barriers are a complicated topic, a common source of confusion in submitted
> code, and their incorrect use is a common cause of bugs.  It *really* doesn't
> help when Xen's API is the same as Linux, but its ABI different.
> 
> Bring the two back in line, so programmers stand a chance of actually getting
> their use correct.
> 
> As Xen has no current need for mandatory barriers, leave them commented out to
> avoid accidential misue.
> 
> No functional change.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
> ---
> CC: Jan Beulich <JBeulich@suse.com>
> ---
>  xen/include/asm-x86/system.h        | 31 +++++++++++++++++++++++--------
>  xen/include/asm-x86/x86_64/system.h |  3 ---
>  2 files changed, 23 insertions(+), 11 deletions(-)
> 
> diff --git a/xen/include/asm-x86/system.h b/xen/include/asm-x86/system.h
> index 9cb6fd7..9cd401a 100644
> --- a/xen/include/asm-x86/system.h
> +++ b/xen/include/asm-x86/system.h
> @@ -164,23 +164,38 @@ static always_inline unsigned long __xadd(
>      ((typeof(*(ptr)))__xadd(ptr, (typeof(*(ptr)))(v), sizeof(*(ptr))))
>  
>  /*
> + * Mandatory barriers, for the ordering of reads and writes with MMIO devices
> + * mapped with reduced cacheability.
> + *
> + * Xen has no such device drivers, and therefore no need for mandatory
> + * barriers.  These these are hidden to avoid their misuse; If a future need

Duplicate "these".


Juergen

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