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>

1 files changed, 7 insertions, 0 deletions

diff --git a/include/hw/boards.h b/include/hw/boards.h
index e46a744bcd..a51da9c440 100644
--- a/include/hw/boards.h
+++ b/include/hw/boards.h
@@ -86,6 +86,12 @@ typedef struct {
  *    Returns a @HotpluggableCPUList, which describes CPUs objects which
  *    could be added with -device/device_add.
  *    Caller is responsible for freeing returned list.
+ * @minimum_page_bits:
+ *    If non-zero, the board promises never to create a CPU with a page size
+ *    smaller than this, so QEMU can use a more efficient larger page
+ *    size than the target architecture's minimum. (Attempting to create
+ *    such a CPU will fail.) Note that changing this is a migration
+ *    compatibility break for the machine.
  */
 struct MachineClass {
     /*< private >*/
@@ -124,6 +130,7 @@ struct MachineClass {
     ram_addr_t default_ram_size;
     bool option_rom_has_mr;
     bool rom_file_has_mr;
+    int minimum_page_bits;
 
     HotplugHandler *(*get_hotplug_handler)(MachineState *machine,
                                            DeviceState *dev);