cpu: Support a target CPU having a variable page size

Support target CPUs having a page size which isn't knownn
at compile time. To use this, the CPU implementation should:
 * define TARGET_PAGE_BITS_VARY
 * not define TARGET_PAGE_BITS
 * define TARGET_PAGE_BITS_MIN to the smallest value it
   might possibly want for TARGET_PAGE_BITS
 * call set_preferred_target_page_bits() in its realize
   function to indicate the actual preferred target page
   size for the CPU (and report any error from it)

In CONFIG_USER_ONLY, the CPU implementation should continue
to define TARGET_PAGE_BITS appropriately for the guest
OS page size.

Machines which want to take advantage of having the page
size something larger than TARGET_PAGE_BITS_MIN must
set the MachineClass minimum_page_bits field to a value
which they guarantee will be no greater than the preferred
page size for any CPU they create.

Note that changing the target page size by setting
minimum_page_bits is a migration compatibility break
for that machine.

For debugging purposes, attempts to use TARGET_PAGE_SIZE
before it has been finally confirmed will assert.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>

exec.c

@@ -93,6 +93,11 @@ static MemoryRegion io_mem_unassigned;
 
 #endif
 
+#ifdef TARGET_PAGE_BITS_VARY
+int target_page_bits;
+bool target_page_bits_decided;
+#endif
+
 struct CPUTailQ cpus = QTAILQ_HEAD_INITIALIZER(cpus);
 /* current CPU in the current thread. It is only valid inside
    cpu_exec() */
@@ -102,8 +107,37 @@ __thread CPUState *current_cpu;
    2 = Adaptive rate instruction counting.  */
 int use_icount;
 
+bool set_preferred_target_page_bits(int bits)
+{
+    /* The target page size is the lowest common denominator for all
+     * the CPUs in the system, so we can only make it smaller, never
+     * larger. And we can't make it smaller once we've committed to
+     * a particular size.
+     */
+#ifdef TARGET_PAGE_BITS_VARY
+    assert(bits >= TARGET_PAGE_BITS_MIN);
+    if (target_page_bits == 0 || target_page_bits > bits) {
+        if (target_page_bits_decided) {
+            return false;
+        }
+        target_page_bits = bits;
+    }
+#endif
+    return true;
+}
+
 #if !defined(CONFIG_USER_ONLY)
 
+static void finalize_target_page_bits(void)
+{
+#ifdef TARGET_PAGE_BITS_VARY
+    if (target_page_bits == 0) {
+        target_page_bits = TARGET_PAGE_BITS_MIN;
+    }
+    target_page_bits_decided = true;
+#endif
+}
+
 typedef struct PhysPageEntry PhysPageEntry;
 
 struct PhysPageEntry {
@@ -2807,6 +2841,14 @@ void cpu_register_map_client(QEMUBH *bh)
 void cpu_exec_init_all(void)
 {
     qemu_mutex_init(&ram_list.mutex);
+    /* The data structures we set up here depend on knowing the page size,
+     * so no more changes can be made after this point.
+     * In an ideal world, nothing we did before we had finished the
+     * machine setup would care about the target page size, and we could
+     * do this much later, rather than requiring board models to state
+     * up front what their requirements are.
+     */
+    finalize_target_page_bits();
     io_mem_init();
     memory_map_init();
     qemu_mutex_init(&map_client_list_lock);