[09/17] x86/PV: use get_unsafe() instead of copy_from_unsafe()

Commit Message

Jan Beulich Jan. 14, 2021, 3:07 p.m. UTC

The former expands to a single (memory accessing) insn, which the latter
does not guarantee. Yet we'd prefer to read consistent PTEs rather than
risking a split read racing with an update done elsewhere.

Signed-off-by: Jan Beulich <[e-mail]>

Patch

--- a/xen/arch/x86/pv/mm.c
+++ b/xen/arch/x86/pv/mm.c
@@ -41,9 +41,7 @@  l1_pgentry_t *map_guest_l1e(unsigned lon
         return NULL;
 
     /* Find this l1e and its enclosing l1mfn in the linear map. */
-    if ( copy_from_unsafe(&l2e,
-                          &__linear_l2_table[l2_linear_offset(linear)],
-                          sizeof(l2_pgentry_t)) )
+    if ( get_unsafe(l2e, &__linear_l2_table[l2_linear_offset(linear)]) )
         return NULL;
 
     /* Check flags that it will be safe to read the l1e. */
--- a/xen/arch/x86/pv/mm.h
+++ b/xen/arch/x86/pv/mm.h
@@ -22,9 +22,7 @@  static inline l1_pgentry_t guest_get_eff
         toggle_guest_pt(curr);
 
     if ( unlikely(!__addr_ok(linear)) ||
-         copy_from_unsafe(&l1e,
-                          &__linear_l1_table[l1_linear_offset(linear)],
-                          sizeof(l1_pgentry_t)) )
+         get_unsafe(l1e, &__linear_l1_table[l1_linear_offset(linear)]) )
         l1e = l1e_empty();
 
     if ( user_mode )