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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Message-ID: <55C21DF3.2090201@citrix.com>
Date:	Wed, 5 Aug 2015 15:30:11 +0100
From:	Julien Grall <julien.grall@...rix.com>
To:	David Vrabel <david.vrabel@...rix.com>,
	<xen-devel@...ts.xenproject.org>
CC:	<linux-kernel@...r.kernel.org>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	<ian.campbell@...rix.com>, <linux-arm-kernel@...ts.infradead.org>,
	<stefano.stabellini@...citrix.com>
Subject: Re: [Xen-devel] [PATCH v2 02/20] xen: Introduce a function to split
 a Linux page into Xen page

Hi David,

On 24/07/15 11:10, David Vrabel wrote:
> 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.

Thinking more about this suggestion. The callback (fn) is getting a 4K
PFN in parameter and not a GFN.

This is because the balloon code seems to require having a 4K PFN in
hand in few places. For instance XENMEM_populate_physmap and
HYPERVISOR_update_va_mapping.

Although, I'm not sure to understand the difference between GMFN, and
GPFN in the hypercall doc.

Regards,

-- 
Julien Grall
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