[PATCH v2] x86/mm: limit deferred TLB flushing to PV domain support

[Roger Pau Monne <roger@xxxxxxxxxxxxxx>]

The current logic on x86 will mark all domain owned pages as needing a TLB
flush before being re-used.  However such TLB flushing is only strictly
needed when the pages might have been mapped by a PV domain, as those can
keep a reference to the page in the TLB after it has been freed.

Limit the flushing to builds with PV domain support, as tracking whether a
page might have been mapped by a PV domain is not trivial (and possibly not
worth the extra logic).

Signed-off-by: Roger Pau Monné <roger@xxxxxxxxxxxxxx>
---
Changes since v1:
 - Only avoid the flush if there's no PV domain support.
 - Fix comment.
---
 xen/common/page_alloc.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c
index 62ac89b824de..cbb2af7f64ce 100644
--- a/xen/common/page_alloc.c
+++ b/xen/common/page_alloc.c
@@ -1538,8 +1538,11 @@ static bool mark_page_free(struct page_info *pg, mfn_t 
mfn)
         BUG();
     }
 
-    /* If a page has no owner it will need no safety TLB flush. */
-    pg->u.free.need_tlbflush = (page_get_owner(pg) != NULL);
+    /*
+     * If a page has no owner and there's no PV domain support it will need no
+     * safety TLB flush, there can be no stale TLB entries.
+     */
+    pg->u.free.need_tlbflush = IS_ENABLED(CONFIG_PV) && page_get_owner(pg);
     if ( pg->u.free.need_tlbflush )
         page_set_tlbflush_timestamp(pg);
 
-- 
2.55.0