c18n: Use superpages to store trampolines

libexec/rtld-elf/rtld_c18n.c

@@ -1021,16 +1021,12 @@ struct tramp_pg {
  _Alignas(_Alignof(void *)) char trampolines[];
 };
 
-/*
- * 64K is the largest size such that any capability-aligned proper
- * sub-range can be exactly bounded by a Morello capability.
- */
-#define C18N_TRAMPOLINE_PAGE_SIZE 64 * 1024
+static size_t tramp_pg_size;
 
 static struct tramp_pg *
 tramp_pg_new(struct tramp_pg *next)
 {
- size_t capacity = C18N_TRAMPOLINE_PAGE_SIZE;
+ size_t capacity = tramp_pg_size;
  struct tramp_pg *pg;
 
  pg = mmap(NULL, capacity, PROT_READ | PROT_WRITE | PROT_EXEC, MAP_ANON,
@@ -1066,7 +1062,7 @@ tramp_pg_push(struct tramp_pg *pg, size_t len)
  */
  &size, n_size, memory_order_relaxed, memory_order_relaxed));
 
- tramp = cheri_setbounds(tramp, len);
+ tramp = cheri_setboundsexact(tramp, len);
  assert(cheri_gettag(tramp));
 
  return (tramp);
@@ -1599,10 +1595,17 @@ c18n_init(Obj_Entry *obj_rtld, Elf_Auxinfo *aux_info[])
  }
 }
 
+/*
+ * Trampoline pages should be large to minimise pressure on the TLB, but not too
+ * large. 4MB is a reasonable threshold.
+ */
+#define MAX_TRAMP_PG_SIZE (4 * 1024 * 1024)
+
 void
 c18n_init2(void)
 {
  uintptr_t sealer;
+ int n = npagesizes;
 
  /*
  * Allocate otypes for RTLD use
@@ -1638,8 +1641,14 @@ c18n_init2(void)
  expand_tramp_table(9);
 
  /*
- * Create the first trampoline page.
+ * Find a suitable page size and create the first trampoline page.
  */
+ while (n-- > 0)
+ if (pagesizes[n] <= MAX_TRAMP_PG_SIZE) {
+ tramp_pg_size = pagesizes[n];
+ break;
+ }
+ assert(tramp_pg_size > 0);
  atomic_store_explicit(&tramp_pgs.head, tramp_pg_new(NULL),
  memory_order_relaxed);
 }