From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Vrabel <david.vrabel@citrix.com>
Subject: Re: [PATCHv2 5/5] evtchn: pad struct evtchn to 64 bytes
Date: Tue, 16 Jun 2015 14:13:09 +0100
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To: Julien Grall <julien.grall.oss@gmail.com>, Jan Beulich <JBeulich@suse.com>, Julien Grall <julien.grall@citrix.com>
Cc: xen-devel@lists.xenproject.org, Ian Campbell <ian.campbell@citrix.com>, Keir Fraser <keir@xen.org>, David Vrabel <david.vrabel@citrix.com>, Tim Deegan <tim@xen.org>
List-Id: xen-devel@lists.xenproject.org

On 16/06/15 13:57, Julien Grall wrote:
> On 16/06/15 12:59, Jan Beulich wrote:
>>>>> On 16.06.15 at 13:14, <julien.grall@citrix.com> wrote:
>>> On 15/06/2015 16:48, David Vrabel wrote:
>>>> diff --git a/xen/include/xen/sched.h b/xen/include/xen/sched.h
>>>> index 44ea92d..a0ff9d2 100644
>>>> --- a/xen/include/xen/sched.h
>>>> +++ b/xen/include/xen/sched.h
>>>> @@ -129,7 +129,7 @@ struct evtchn
>>>>   #endif
>>>>       } ssid;
>>>>   #endif
>>>> -};
>>>> +} __attribute__((aligned(64)));
>>>
>>> Why don't you use __cacheline_aligned?
>>
>> That would double the size on x86, for little or no benefit.
> 
> Well, the cache line size is not necessarily 64 bytes on every
> architecture. In the case of ARM, the cache line depends on the
> processor version.
> 
> __cacheline_aligned is the only way to ensure that the cache line is not
> shared on ARM.
> 
> AFAIU, the goal of this patch is to avoid sharing the cache line. If
> not, the commit message is misleading because it claims that a cache
> line is always 64 bytes...

We want to avoid sharing the cache line where we can do so for no
additional memory cost.

David