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Re: [Xen-devel] Error restoring DomU when using GPLPV

To: James Harper <james.harper@xxxxxxxxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Error restoring DomU when using GPLPV
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Tue, 04 Aug 2009 12:02:12 +0100
Cc: Joshua West <jwest@xxxxxxxxxxxx>
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On 04/08/2009 11:40, "James Harper" <james.harper@xxxxxxxxxxxxxxxx> wrote:

>> Oh yes, there is no direct opposite of add_to_physmap... But I think
>> decrease_reservation will work okay in this case, fortunately.
> Given that I'm not going to use the grant table subsequent to unmapping
> them I'll probably get away with it, but does
> XENMEM_decrease_reservation actually tell xen that the pages are no
> longer actually part of the grant table?

No, for a xenheap page the page won't actually get freed. Xen keeps a
reference to them until the domain is finally destroyed.

Regarding the Linux PV-on-HVM drivers - they may have the same issue. Full
PV guests do not as they have a gnttab_suspend() function called during
suspend callback (and for subtle reasons xc_domain_save can detect and not
save Xenheap pages for a full PV guests anyway - because it can see the P2M
table in that case).

Like I said before -- unmapping the gnttab pages I think will not help you
for live migration, but I suppose it is a reasonable thing to do anyway. For
live migration I think xc_domain_save needs t get a bit smarter about
Xenheap pages in HVM guests.

 -- Keir

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