lguest: map Switcher text whenever we allocate a new pagetable.

It's always to same, so no need to put in the PTE every time we're
about to run.  Keep a flag to track whether the pagetable has the
Switcher entries allocated, and when allocating always initialize the
Switcher text PTE.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

2 files changed, 33 insertions, 10 deletions

diff --git a/drivers/lguest/lg.h b/drivers/lguest/lg.h
index faac9fc6db22..005929a3fd52 100644
--- a/drivers/lguest/lg.h
+++ b/drivers/lguest/lg.h
@@ -16,6 +16,7 @@
 
 struct pgdir {
         unsigned long gpgdir;
+        bool switcher_mapped;
         pgd_t *pgdir;
 };
 
diff --git a/drivers/lguest/page_tables.c b/drivers/lguest/page_tables.c
index 1f48f2712f3a..d1a5de45be02 100644
--- a/drivers/lguest/page_tables.c
+++ b/drivers/lguest/page_tables.c
@@ -736,18 +736,39 @@ static unsigned int new_pgdir(struct lg_cpu *cpu,
 
 /*H:501
  * We do need the Switcher code mapped at all times, so we allocate that
- * part of the Guest page table here, and populate it when we're about to run
- * the guest.
+ * part of the Guest page table here.  We map the Switcher code immediately,
+ * but defer mapping of the guest register page and IDT/LDT etc page until
+ * just before we run the guest in map_switcher_in_guest().
+ *
+ * We *could* do this setup in map_switcher_in_guest(), but at that point
+ * we've interrupts disabled, and allocating pages like that is fraught: we
+ * can't sleep if we need to free up some memory.
  */
 static bool allocate_switcher_mapping(struct lg_cpu *cpu)
 {
         int i;
 
         for (i = 0; i < TOTAL_SWITCHER_PAGES; i++) {
-                if (!find_spte(cpu, switcher_addr + i * PAGE_SIZE, true,
-                               CHECK_GPGD_MASK, _PAGE_TABLE))
+                pte_t *pte = find_spte(cpu, switcher_addr + i * PAGE_SIZE, true,
+                                       CHECK_GPGD_MASK, _PAGE_TABLE);
+                if (!pte)
                         return false;
+
+                /*
+                 * Map the switcher page if not already there.  It might
+                 * already be there because we call allocate_switcher_mapping()
+                 * in guest_set_pgd() just in case it did discard our Switcher
+                 * mapping, but it probably didn't.
+                 */
+                if (i == 0 && !(pte_flags(*pte) & _PAGE_PRESENT)) {
+                        /* Get a reference to the Switcher page. */
+                        get_page(lg_switcher_pages[0]);
+                        /* Create a read-only, exectuable, kernel-style PTE */
+                        set_pte(pte,
+                                mk_pte(lg_switcher_pages[0], PAGE_KERNEL_RX));
+                }
         }
+        cpu->lg->pgdirs[cpu->cpu_pgd].switcher_mapped = true;
         return true;
 }
 
@@ -768,6 +789,7 @@ static void release_all_pagetables(struct lguest *lg)
                 /* Every PGD entry. */
                 for (j = 0; j < PTRS_PER_PGD; j++)
                         release_pgd(lg->pgdirs[i].pgdir + j);
+                lg->pgdirs[i].switcher_mapped = false;
         }
 }
 
@@ -827,8 +849,10 @@ void guest_new_pagetable(struct lg_cpu *cpu, unsigned long pgtable)
         if (repin)
                 pin_stack_pages(cpu);
 
-        if (!allocate_switcher_mapping(cpu))
-                kill_guest(cpu, "Cannot populate switcher mapping");
+        if (!cpu->lg->pgdirs[cpu->cpu_pgd].switcher_mapped) {
+                if (!allocate_switcher_mapping(cpu))
+                        kill_guest(cpu, "Cannot populate switcher mapping");
+        }
 }
 /*:*/
 
@@ -1076,10 +1100,8 @@ void map_switcher_in_guest(struct lg_cpu *cpu, struct lguest_pages *pages)
         struct page *percpu_switcher_page, *regs_page;
         pte_t *pte;
 
-        /* Code page should always be mapped, and executable. */
-        pte = find_spte(cpu, switcher_addr, false, 0, 0);
-        get_page(lg_switcher_pages[0]);
-        set_pte(pte, mk_pte(lg_switcher_pages[0], PAGE_KERNEL_RX));
+        /* Switcher page should always be mapped! */
+        BUG_ON(!cpu->lg->pgdirs[cpu->cpu_pgd].switcher_mapped);
 
         /* Clear all the Switcher mappings for any other CPUs. */
         /* FIXME: This is dumb: update only when Host CPU changes. */