1 files changed, 25 insertions, 5 deletions

diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
index 3a73785631ce..3f1783a79a3c 100644
--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -308,8 +308,20 @@ static bool xen_batched_set_pte(pte_t *ptep, pte_t pteval)
 
 static inline void __xen_set_pte(pte_t *ptep, pte_t pteval)
 {
-        if (!xen_batched_set_pte(ptep, pteval))
-                native_set_pte(ptep, pteval);
+        if (!xen_batched_set_pte(ptep, pteval)) {
+                /*
+                 * Could call native_set_pte() here and trap and
+                 * emulate the PTE write but with 32-bit guests this
+                 * needs two traps (one for each of the two 32-bit
+                 * words in the PTE) so do one hypercall directly
+                 * instead.
+                 */
+                struct mmu_update u;
+
+                u.ptr = virt_to_machine(ptep).maddr | MMU_NORMAL_PT_UPDATE;
+                u.val = pte_val_ma(pteval);
+                HYPERVISOR_mmu_update(&u, 1, NULL, DOMID_SELF);
+        }
 }
 
 static void xen_set_pte(pte_t *ptep, pte_t pteval)
@@ -1416,13 +1428,21 @@ static pte_t __init mask_rw_pte(pte_t *ptep, pte_t pte)
 }
 #endif /* CONFIG_X86_64 */
 
-/* Init-time set_pte while constructing initial pagetables, which
-   doesn't allow RO pagetable pages to be remapped RW */
+/*
+ * Init-time set_pte while constructing initial pagetables, which
+ * doesn't allow RO page table pages to be remapped RW.
+ *
+ * Many of these PTE updates are done on unpinned and writable pages
+ * and doing a hypercall for these is unnecessary and expensive.  At
+ * this point it is not possible to tell if a page is pinned or not,
+ * so always write the PTE directly and rely on Xen trapping and
+ * emulating any updates as necessary.
+ */
 static void __init xen_set_pte_init(pte_t *ptep, pte_t pte)
 {
         pte = mask_rw_pte(ptep, pte);
 
-        xen_set_pte(ptep, pte);
+        native_set_pte(ptep, pte);
 }
 
 static void pin_pagetable_pfn(unsigned cmd, unsigned long pfn)