[PATCH 2/5] x86/mm: allow a privileged PV domain to map guest mfns

[Paul Durrant <paul.durrant@citrix.com>]

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.

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@citrix.com>
---
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
---
 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 f8b3505849..6bf8945406 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -1010,12 +1010,15 @@ 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


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