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Date: Wed, 21 May 2008 12:53:52 -0500
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] Re: [PATCH][v2] Align file accesses with cache=off
	(O_DIRECT)
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To: Andrea Arcangeli <andrea@qumranet.com>
Cc: Blue Swirl <blauwirbel@gmail.com>, Laurent Vivier <Laurent.Vivier@bull.net>, qemu-devel@nongnu.org, Paul Brook <paul@codesourcery.com>

Andrea Arcangeli wrote:
> On Wed, May 21, 2008 at 08:18:01PM +0300, Avi Kivity wrote:
>   
>> Yes, that's the reason.  Here zerocopy is not the motivation; instead, we 
>> have host-cached pages that are used directly in the guest.  So we get both 
>> reduced memory footprint, and host caching.  O_DIRECT reduces the memory 
>> footprint but kills host caching.
>>     
>
> Sure. So MAP_SHARED+remap_file_pages should work just fine to achieve
> zerocopy I/O.
>   

MAP_SHARED cannot be done transparently to the guest, that's the 
motivating reason behind MAP_PRIVATE.

Regards,

Anthony Liguori