[xen master] x86: don't override INVALID_M2P_ENTRY with SHARED_M2P_ENTRY

[patchbot@xxxxxxx]

commit c9476c4ad72e8a1842d713c74843034c7ec6eb51
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Fri Sep 11 14:14:43 2020 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Fri Sep 11 14:14:43 2020 +0200

    x86: don't override INVALID_M2P_ENTRY with SHARED_M2P_ENTRY
    
    While in most cases code ahead of the invocation of set_gpfn_from_mfn()
    deals with shared pages, at least in set_typed_p2m_entry() I can't spot
    such handling (it's entirely possible there's code missing there). Let's
    try to play safe and add an extra check.
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 xen/arch/x86/x86_64/mm.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/x86_64/mm.c b/xen/arch/x86/x86_64/mm.c
index 1f32062c15..0d1aadbfce 100644
--- a/xen/arch/x86/x86_64/mm.c
+++ b/xen/arch/x86/x86_64/mm.c
@@ -1327,12 +1327,19 @@ destroy_frametable:
 
 void set_gpfn_from_mfn(unsigned long mfn, unsigned long pfn)
 {
-    const struct domain *d = page_get_owner(mfn_to_page(_mfn(mfn)));
-    unsigned long entry = (d && (d == dom_cow)) ? SHARED_M2P_ENTRY : pfn;
+    unsigned long entry = pfn;
 
     if ( unlikely(!machine_to_phys_mapping_valid) )
         return;
 
+    if ( entry != INVALID_M2P_ENTRY )
+    {
+        const struct domain *d = page_get_owner(mfn_to_page(_mfn(mfn)));
+
+        if ( d && (d == dom_cow) )
+            entry = SHARED_M2P_ENTRY;
+    }
+
     if ( opt_pv32 &&
          mfn < (RDWR_COMPAT_MPT_VIRT_END - RDWR_COMPAT_MPT_VIRT_START) / 4 )
         compat_machine_to_phys_mapping[mfn] = entry;
--
generated by git-patchbot for /home/xen/git/xen.git#master