[PATCH v2 4/7] x86: extend mod_l1_entry() to optionally return the old PTE value

[Kevin Lampis <kevin.lampis@xxxxxxxxxx>]

Calling mod_l1_entry() with a valid ol1e_out pointer will do an atomic xchg to
set the new pte value and return the old pte value through the ol1e_out
pointer. If the ol1e_out pointer is NULL then old behavior is preserved.

Signed-off-by: Kevin Lampis <kevin.lampis@xxxxxxxxxx>
---
Changes in v2:
- UPDATE_ENTRY now returns the old pte value instead of using an out
  pointer
---
 xen/arch/x86/mm.c | 23 +++++++++++++++++------
 1 file changed, 17 insertions(+), 6 deletions(-)

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index dcbc44f57c54..59ad5ce16486 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -2150,7 +2150,8 @@ static void l3t_unlock(struct page_info *page)
 /* Update the L1 entry at pl1e to new value nl1e. */
 static int mod_l1_entry(l1_pgentry_t *pl1e, l1_pgentry_t nl1e,
                         mfn_t gl1mfn, unsigned int update_flags,
-                        struct vcpu *pt_vcpu, struct domain *pg_dom)
+                        struct vcpu *pt_vcpu, struct domain *pg_dom,
+                        l1_pgentry_t *ol1e_out)
 {
     l1_pgentry_t ol1e = l1e_read(pl1e);
     struct domain *pt_dom = pt_vcpu->domain;
@@ -2211,9 +2212,12 @@ static int mod_l1_entry(l1_pgentry_t *pl1e, l1_pgentry_t 
nl1e,
         /* Fast path for sufficiently-similar mappings. */
         if ( !l1e_has_changed(ol1e, nl1e, ~FASTPATH_FLAG_WHITELIST) )
         {
-            UPDATE_ENTRY(l1, pl1e, ol1e, nl1e, gl1mfn, pt_vcpu, update_flags);
+            ol1e.l1 = UPDATE_ENTRY(l1, pl1e, ol1e, nl1e, gl1mfn, pt_vcpu,
+                                   update_flags);
             if ( page )
                 put_page(page);
+            if ( ol1e_out )
+                *ol1e_out = ol1e;
             return 0;
         }
 
@@ -2234,14 +2238,20 @@ static int mod_l1_entry(l1_pgentry_t *pl1e, 
l1_pgentry_t nl1e,
         if ( page )
             put_page(page);
 
-        UPDATE_ENTRY(l1, pl1e, ol1e, nl1e, gl1mfn, pt_vcpu, update_flags);
+        ol1e.l1 = UPDATE_ENTRY(l1, pl1e, ol1e, nl1e, gl1mfn, pt_vcpu,
+                               update_flags);
     }
     else if ( pv_l1tf_check_l1e(pt_dom, nl1e) )
         return -ERESTART;
     else
-        UPDATE_ENTRY(l1, pl1e, ol1e, nl1e, gl1mfn, pt_vcpu, update_flags);
+        ol1e.l1 = UPDATE_ENTRY(l1, pl1e, ol1e, nl1e, gl1mfn, pt_vcpu,
+                               update_flags);
 
     put_page_from_l1e(ol1e, pt_dom);
+
+    if ( !rc && ol1e_out )
+        *ol1e_out = ol1e;
+
     return rc;
 }
 
@@ -4109,7 +4119,7 @@ long do_mmu_update(
                 {
                 case PGT_l1_page_table:
                     rc = mod_l1_entry(va, l1e_from_intpte(req.val), mfn,
-                                      update_flags, v, pg_owner);
+                                      update_flags, v, pg_owner, NULL);
                     break;
 
                 case PGT_l2_page_table:
@@ -4474,7 +4484,8 @@ static int __do_update_va_mapping(
         goto out;
     }
 
-    rc = mod_l1_entry(pl1e, val, gl1mfn, MMU_NORMAL_PT_UPDATE, v, pg_owner);
+    rc = mod_l1_entry(pl1e, val, gl1mfn, MMU_NORMAL_PT_UPDATE, v, pg_owner,
+                      NULL);
 
     page_unlock(gl1pg);
     put_page(gl1pg);
-- 
2.52.0