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

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Date:   Tue, 18 Oct 2016 18:08:31 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:     david.vrabel@...rix.com, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org, roger.pau@...rix.com
Subject: Re: [Xen-devel] [PATCH 6/8] xen/pvh: Initialize grant table for PVH
 guests

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().

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


Juergen