xen: remove scratch frames for ballooned pages and m2p override

The scratch frame mappings for ballooned pages and the m2p override
are broken.  Remove them in preparation for replacing them with
simpler mechanisms that works.

The scratch pages did not ensure that the page was not in use.  In
particular, the foreign page could still be in use by hardware.  If
the guest reused the frame the hardware could read or write that
frame.

The m2p override did not handle the same frame being granted by two
different grant references.  Trying an M2P override lookup in this
case is impossible.

With the m2p override removed, the grant map/unmap for the kernel
mappings (for x86 PV) can be easily batched in
set_foreign_p2m_mapping() and clear_foreign_p2m_mapping().

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>

1 files changed, 4 insertions, 14 deletions

diff --git a/arch/x86/include/asm/xen/page.h b/arch/x86/include/asm/xen/page.h
index e9f52fe2d56a..358dcd338915 100644
--- a/arch/x86/include/asm/xen/page.h
+++ b/arch/x86/include/asm/xen/page.h
@@ -57,7 +57,6 @@ extern int set_foreign_p2m_mapping(struct gnttab_map_grant_ref *map_ops,
 extern int clear_foreign_p2m_mapping(struct gnttab_unmap_grant_ref *unmap_ops,
                                      struct gnttab_unmap_grant_ref *kunmap_ops,
                                      struct page **pages, unsigned int count);
-extern unsigned long m2p_find_override_pfn(unsigned long mfn, unsigned long pfn);
 
 /*
  * Helper functions to write or read unsigned long values to/from
@@ -154,21 +153,12 @@ static inline unsigned long mfn_to_pfn(unsigned long mfn)
                 return mfn;
 
         pfn = mfn_to_pfn_no_overrides(mfn);
-        if (__pfn_to_mfn(pfn) != mfn) {
-                /*
-                 * If this appears to be a foreign mfn (because the pfn
-                 * doesn't map back to the mfn), then check the local override
-                 * table to see if there's a better pfn to use.
-                 *
-                 * m2p_find_override_pfn returns ~0 if it doesn't find anything.
-                 */
-                pfn = m2p_find_override_pfn(mfn, ~0);
-        }
+        if (__pfn_to_mfn(pfn) != mfn)
+                pfn = ~0;
 
         /*
-         * pfn is ~0 if there are no entries in the m2p for mfn or if the
-         * entry doesn't map back to the mfn and m2p_override doesn't have a
-         * valid entry for it.
+         * pfn is ~0 if there are no entries in the m2p for mfn or the
+         * entry doesn't map back to the mfn.
          */
         if (pfn == ~0 && __pfn_to_mfn(mfn) == IDENTITY_FRAME(mfn))
                 pfn = mfn;