linux-kernel - Re: [Xen-devel] [PATCH v2 02/20] xen: Introduce a function to split a Linux page into Xen page

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Date:	Fri, 24 Jul 2015 11:10:39 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Julien Grall <julien.grall@...rix.com>,
	<xen-devel@...ts.xenproject.org>
CC:	<ian.campbell@...rix.com>, <stefano.stabellini@...citrix.com>,
	<linux-kernel@...r.kernel.org>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [Xen-devel] [PATCH v2 02/20] xen: Introduce a function to split
 a Linux page into Xen page

On 24/07/15 10:54, Julien Grall wrote:
> On 24/07/15 10:31, David Vrabel wrote:
>> On 09/07/15 21:42, Julien Grall wrote:
>>> The Xen interface is always using 4KB page. This means that a Linux page
>>> may be split across multiple Xen page when the page granularity is not
>>> the same.
>>>
>>> This helper will break down a Linux page into 4KB chunk and call the
>>> helper on each of them.
>> [...]
>>> --- a/include/xen/page.h
>>> +++ b/include/xen/page.h
>>> @@ -39,4 +39,24 @@ struct xen_memory_region xen_extra_mem[XEN_EXTRA_MEM_MAX_REGIONS];
>>>  
>>>  extern unsigned long xen_released_pages;
>>>  
>>> +typedef int (*xen_pfn_fn_t)(struct page *page, unsigned long pfn, void *data);
>>> +
>>> +/* Break down the page in 4KB granularity and call fn foreach xen pfn */
>>> +static inline int xen_apply_to_page(struct page *page, xen_pfn_fn_t fn,
>>> +				    void *data)
>>
>> I think this should be outlined (unless you have measurements that
>> support making it inlined).
> 
> I don't have any performance measurements. Although, when Linux is using
> 4KB page granularity, the loop in this helper will be dropped by the
> helper. The code would look like:
> 
> unsigned long pfn = xen_page_to_pfn(page);
> 
> ret = fn(page, fn, data);
> if (ret)
>   return ret;
> 
> The compiler could even inline the callback (fn). So it drops 2
> functions call.

Ok, keep it inlined.

>> Also perhaps make it
>>
>> int xen_for_each_gfn(struct page *page,
>>                      xen_gfn_fn_t fn, void *data);
> 
> gfn standing for Guest Frame Number right?

Yes.  This suggestion is just changing the name to make it more obvious
what it does.

David
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