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

[Daniel De Graaf <dgdegra@xxxxxxxxxxxxx>]

On 10/24/2011 06:08 PM, Konrad Rzeszutek Wilk wrote:
> On Mon, Oct 24, 2011 at 05:47:40PM -0400, Daniel De Graaf wrote:
>> 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.
> 
> Uh, aren't pages from alloc_page ("if the balloon does not have any sitting 
> around
> for us") obtained from normal memory that is allocated at startup. And at 
> startup
> those swaths of memory are obtained by populate_physmap call?
> 

Yes, but alloc_xenballooned_pages calls XENMEM_decrease_reservation to remove 
the
MFN mappings for these pages, so they are returned to the state where 
populate_physmap
has not been called on them.

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