[PATCH v2] x86/p2m: fix xenmem_add_to_physmap_one double page removal

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

If the new gfn matches the previous one (ie: gpfn == old_gpfn)
xenmem_add_to_physmap_one will issue a duplicated call to
guest_physmap_remove_page with the same guest frame number, because
the get_gpfn_from_mfn call has been moved by commit f8582da041 to be
performed before the original page is removed. This leads to the
second guest_physmap_remove_page failing, which was not the case
before commit f8582da041.

Add a shortcut to skip modifying the p2m if the mapping is already as
requested.

Fixes: f8582da041 ('x86/mm: pull a sanity check earlier in 
xenmem_add_to_physmap_one()')
Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
Changes since v1:
 - Short circuit and skip modifying the p2m.
---
 xen/arch/x86/mm/p2m.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/xen/arch/x86/mm/p2m.c b/xen/arch/x86/mm/p2m.c
index 674a6f4fe9..bcdc5c7014 100644
--- a/xen/arch/x86/mm/p2m.c
+++ b/xen/arch/x86/mm/p2m.c
@@ -2799,6 +2799,13 @@ int xenmem_add_to_physmap_one(
         goto put_all;
     }
 
+    if ( gfn_eq(_gfn(old_gpfn), gpfn) )
+    {
+        /* Nothing to do, mapping is already as requested. */
+        ASSERT(mfn_eq(prev_mfn, mfn));
+        goto put_all;
+    }
+
     /* Remove previously mapped page if it was present. */
     if ( p2mt == p2m_mmio_direct )
         rc = -EPERM;
-- 
2.33.0