[xen staging] x86: XENMAPSPACE_gmfn{,_batch,_range} want to special case idx == gpfn

[patchbot@xxxxxxx]

commit ba45ae4d7f3a93ee7297d24bb03a564146d321c6
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Fri Oct 23 10:05:29 2020 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Fri Oct 23 10:05:29 2020 +0200

    x86: XENMAPSPACE_gmfn{,_batch,_range} want to special case idx == gpfn
    
    In this case up to now we've been freeing the page (through
    guest_remove_page(), with the actual free typically happening at the
    put_page() later in the function), but then failing the call on the
    subsequent GFN consistency check. However, in my opinion such a request
    should complete as an "expensive" no-op (leaving aside the potential
    unsharing of the page).
    
    This points out that f33d653f46f5 ("x86: replace bad ASSERT() in
    xenmem_add_to_physmap_one()" would really have needed an XSA, despite
    its description claiming otherwise, as in release builds we then put in
    place a P2M entry referencing the about to be freed page.
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 xen/arch/x86/mm.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index 918ee2bbe3..6d2262a3f0 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -4599,7 +4599,7 @@ int xenmem_add_to_physmap_one(
         if ( is_special_page(mfn_to_page(prev_mfn)) )
             /* Special pages are simply unhooked from this phys slot. */
             rc = guest_physmap_remove_page(d, gpfn, prev_mfn, PAGE_ORDER_4K);
-        else
+        else if ( !mfn_eq(mfn, prev_mfn) )
             /* Normal domain memory is freed, to avoid leaking memory. */
             rc = guest_remove_page(d, gfn_x(gpfn));
     }
--
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