xen/arm: mm: Avoid flushing the TLBs when mapping are inserted

Currently, the function xen_pt_update() will flush the TLBs even when
the mappings are inserted. This is a bit wasteful because we don't
allow mapping replacement. Even if we were, the flush would need to
happen earlier because mapping replacement should use Break-Before-Make
when updating the entry.

A single call to xen_pt_update() can perform a single action. IOW, it
is not possible to, for instance, mix inserting and removing mappings.
Therefore, we can use `flags` to determine what action is performed.

This change will be particularly help to limit the impact of switching
boot time mapping to use xen_pt_update().

Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Luca Fancellu <luca.fancellu@arm.com>
Tested-by: Luca Fancellu <luca.fancellu@arm.com>

xen/arch/arm/mm.c

@@ -1119,7 +1119,13 @@ static bool xen_pt_check_entry(lpae_t entry, mfn_t mfn, unsigned int level,
         /* We should be here with a valid MFN. */
         ASSERT(!mfn_eq(mfn, INVALID_MFN));
 
-        /* We don't allow replacing any valid entry. */
+        /*
+         * We don't allow replacing any valid entry.
+         *
+         * Note that the function xen_pt_update() relies on this
+         * assumption and will skip the TLB flush. The function will need
+         * to be updated if the check is relaxed.
+         */
         if ( lpae_is_valid(entry) )
         {
             if ( lpae_is_mapping(entry, level) )
@@ -1434,11 +1440,16 @@ static int xen_pt_update(unsigned long virt,
     }
 
     /*
-     * Flush the TLBs even in case of failure because we may have
+     * The TLBs flush can be safely skipped when a mapping is inserted
+     * as we don't allow mapping replacement (see xen_pt_check_entry()).
+     *
+     * For all the other cases, the TLBs will be flushed unconditionally
+     * even if the mapping has failed. This is because we may have
      * partially modified the PT. This will prevent any unexpected
      * behavior afterwards.
      */
-    flush_xen_tlb_range_va(virt, PAGE_SIZE * nr_mfns);
+    if ( !((flags & _PAGE_PRESENT) && !mfn_eq(mfn, INVALID_MFN)) )
+        flush_xen_tlb_range_va(virt, PAGE_SIZE * nr_mfns);
 
     spin_unlock(&xen_pt_lock);