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Re: [Xen-devel] [PATCH v2 REPOST 02/12] x86/mm: allow a privileged PV domain to map guest mfns

On Fri, Aug 25, 2017 at 11:05:54AM +0100, Paul Durrant wrote:
> > -----Original Message-----
> > From: Wei Liu [mailto:wei.liu2@xxxxxxxxxx]
> > Sent: 24 August 2017 17:33
> > To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> > Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; Andrew Cooper
> > <Andrew.Cooper3@xxxxxxxxxx>; Jan Beulich <jbeulich@xxxxxxxx>; Wei Liu
> > <wei.liu2@xxxxxxxxxx>
> > Subject: Re: [Xen-devel] [PATCH v2 REPOST 02/12] x86/mm: allow a
> > privileged PV domain to map guest mfns
> > 
> > On Tue, Aug 22, 2017 at 03:50:56PM +0100, Paul Durrant wrote:
> > > In the case where a PV domain is mapping guest resources then it needs
> > make
> > > the HYPERVISOR_mmu_update call using DOMID_SELF, rather than the
> > guest
> > > domid, so that the passed in gmfn values are correctly treated as mfns
> > > rather than gfns present in the guest p2m.
> > >
> > 
> > What would be the callchain like in this case?

> It's exactly like foreign mapping but passing DOMID_SELF. I.e. in
> privcmd (in a PV domain) you have an mfn in your hand that already
> belongs to you rather than the gmfn of a foreign domain.
> > 
> > I don't quite understand how this fits with the resource mapping
> > code in this series.
> > 
> Because (for a PV caller) mapping a resource gives you back mfns that
> are assigned to the calling domain, and the most convenient way of
> using them is to use the existing code that normally deals with priv
> mapping from a foreign domain, but just allow it to use DOMID_SELF.
> This patch is all that's required to make that work.

So the use case is as followed for PV guests:

1. A guest calls acquire_resource to obtain a list of mfns
2. The guest calls the foreign map API to map those mfns into its own
   address space via HYPERVISOR_mmu_update

The mfns belong to the guest itself.

In get_page_from_l1e, l1e contains a valid mfn, real_pg_owner is the
real owner of the page, pg_owner is the nominally owner of the page.
Shouldn't they be the same domain? I'm still quite baffled how you
manage to hit that place.

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