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

[Daniel De Graaf <dgdegra@xxxxxxxxxxxxx>]

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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