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: <55B20B56.7020605@citrix.com>
Date:	Fri, 24 Jul 2015 10:54:30 +0100
From:	Julien Grall <julien.grall@...rix.com>
To:	David Vrabel <david.vrabel@...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 10:31, 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.

> 
> 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?

> or
> 
> int xen_for_each_gfn(struct page **page, unsigned int count,
>                      xen_gfn_fn_t fn, void *data);

count being the number of Linux page or Xen page? We have some code (see
xlate_mmu patch #19) requiring to iter on a specific number of Xen page.
I was thinking to introduce a separate function for iterating on a
specific number of Xen PFN.

We don't want to introduce it for everyone as we need to hide this
complexity from most the caller.

In general case, the 2 suggestions would not be very useful. Most of the
time we have some actions to do per Linux page (see the balloon code for
instance).

Regards,

-- 
Julien Grall
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