Re: [Xen-devel] [PATCH 6/8] xen/pvh: Initialize grant table for PVH guests

From: Boris Ostrovsky
Date: Tue Oct 18 2016 - 12:39:28 EST

On 10/18/2016 12:08 PM, Juergen Gross wrote:
> On 14/10/16 22:02, Boris Ostrovsky wrote:
>> On 10/14/2016 03:51 PM, Konrad Rzeszutek Wilk wrote:
>>> On Fri, Oct 14, 2016 at 03:43:19PM -0400, Boris Ostrovsky wrote:
>>>> On 10/14/2016 03:19 PM, Konrad Rzeszutek Wilk wrote:
>>>>> On Fri, Oct 14, 2016 at 02:05:16PM -0400, Boris Ostrovsky wrote:
>>>>> Perhaps add in here:
>>>>> PVH is like PV in that there are no PCI devices - which HVM
>>>>> code would piggyback on to find the Xen PCI platform device and
>>>>> use its MMIO space to stash the grants in.
>>>>> For PVH we balloon out memory and stash the grants in there.
>>>>> (Which begs the next question - where and when do we balloon out the
>>>>> normal memory back in?)
>>>> Are you saying that we should get back memory that we gave to grant tables?
>>> Yes.
>>> In pure HVM that area is MMIO - which hvmloader has balloonned out.
>>> The hvmloader then balloons that number of pages back at the end of
>>> guest memory (after 4GB).
>> We don't do this for PV though, do we?
> Uuh, kind of. We try to allocate granted pages from the ballooned area.
> See gnttab_alloc_pages().

I meant that we don't give memory back for PV.

> So for PV(H) we don't need to balloon this memory back in as it was
> never shadowed by a grant.

Is it *never* or *may or may not be* shadowed? (I assume "shadowed"
means "used for" here.)