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

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----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.

  Paul

> > This patch removes a check which currently disallows mapping of a page
> when
> > the owner of the page tables matches the domain passed to
> > HYPERVISOR_mmu_update, but that domain is not the real owner of the
> page.
> > The check was introduced by patch d3c6a215ca9 ("x86: Clean up
> > get_page_from_l1e() to correctly distinguish between owner-of-pte and
> > owner-of-data-page in all cases") but it's not clear why it was needed.
> >
> > Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
> > ---
> > Cc: Jan Beulich <jbeulich@xxxxxxxx>
> > Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > ---
> >  xen/arch/x86/mm.c | 13 ++++++++-----
> >  1 file changed, 8 insertions(+), 5 deletions(-)
> >
> > diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
> > index 0abb1e284f..aaa9ff5197 100644
> > --- a/xen/arch/x86/mm.c
> > +++ b/xen/arch/x86/mm.c
> > @@ -989,12 +989,15 @@ get_page_from_l1e(
> >                     (real_pg_owner != dom_cow) ) )
> >      {
> >          /*
> > -         * Let privileged domains transfer the right to map their target
> > -         * domain's pages. This is used to allow stub-domain pvfb export to
> > -         * dom0, until pvfb supports granted mappings. At that time this
> > -         * minor hack can go away.
> > +         * If the real page owner is not the domain specified in the
> > +         * hypercall then establish that the specified domain has
> > +         * mapping privilege over the page owner.
> > +         * This is used to allow stub-domain pvfb export to dom0. It is
> > +         * also used to allow a privileged PV domain to map mfns using
> > +         * DOMID_SELF, which is needed for mapping guest resources such
> > +         * grant table frames.
> >           */
> > -        if ( (real_pg_owner == NULL) || (pg_owner == l1e_owner) ||
> > +        if ( (real_pg_owner == NULL) ||
> >               xsm_priv_mapping(XSM_TARGET, pg_owner, real_pg_owner) )
> >          {
> >              gdprintk(XENLOG_WARNING,
> > --
> > 2.11.0
> >
> >
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxx
> > https://lists.xen.org/xen-devel

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