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, 10 insertions, 0 deletions

diff --git a/vl.c b/vl.c
index 44e08b4fc0..4ec8120834 100644
--- a/vl.c
+++ b/vl.c
@@ -4088,6 +4088,16 @@ int main(int argc, char **argv, char **envp)
     }
     object_property_add_child(object_get_root(), "machine",
                               OBJECT(current_machine), &error_abort);
+
+    if (machine_class->minimum_page_bits) {
+        if (!set_preferred_target_page_bits(machine_class->minimum_page_bits)) {
+            /* This would be a board error: specifying a minimum smaller than
+             * a target's compile-time fixed setting.
+             */
+            g_assert_not_reached();
+        }
+    }
+
     cpu_exec_init_all();
 
     if (machine_class->hw_version) {