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

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 19/10/11 16:01, Daniel De Graaf wrote:
> On 10/19/2011 05:04 AM, Ian Campbell wrote:
>> On Tue, 2011-10-18 at 21:26 +0100, Daniel De Graaf wrote:
>>> For proper grant mappings, HVM guests require pages allocated using
>>> alloc_xenballooned_pages instead of alloc_vm_area.
>>>
>>> Signed-off-by: Daniel De Graaf <dgdegra@xxxxxxxxxxxxx>
>>> ---
[...]
>> Could you also explain where the requirement to use xenballooned pages
>> and not alloc_vm_area comes from in your commit message.
> 
> (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
> obtained from alloc_xenballooned_pages have valid GFNs not currently
> mapped to an MFN, so are available to be used in grant mappings.

Why doesn't (or can't?) Xen add new entries to the GFN-to-MFN map?  Or
why hasn't it reserved a range of GFNs in the map for this?

David

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