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Re: [Xen-devel] [PATCH 1/5] xen/netback: Use xenballooned pages for comm

To: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH 1/5] xen/netback: Use xenballooned pages for comms
From: Daniel De Graaf <dgdegra@xxxxxxxxxxxxx>
Date: Mon, 24 Oct 2011 17:47:40 -0400
Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Ian Campbell <Ian.Campbell@xxxxxxxxxx>, David Vrabel <david.vrabel@xxxxxxxxxx>
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On 10/24/2011 05:40 PM, Konrad Rzeszutek Wilk wrote:
>> (Will move to commit message). In PV guests, it is sufficient to only
>> reserve kernel address space for grant mappings because Xen modifies the
>> mappings directly. HVM guests require that Xen modify the GFN-to-MFN
>> mapping, so the pages being remapped must already be allocated. Pages
> 
> By allocated you mean the populate_physmap hypercall must happen before
> the grant operations are done?
> 
> (When I see allocated I think alloc_page, which I believe is _not_ what
> you were saying).

The pages must be valid kernel pages (with GFNs) which are actually obtained
with alloc_page if the balloon doesn't have any sitting around for us. They
must also *not* be populated in the physmap, which is why we grab them from
the balloon and not from alloc_page directly.

> 
>> obtained from alloc_xenballooned_pages have valid GFNs not currently
>> mapped to an MFN, so are available to be used in grant mappings.
>>
> 

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