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>

3 files changed, 28 insertions, 0 deletions

diff --git a/include/exec/cpu-all.h b/include/exec/cpu-all.h
index b6a705982f..861260d3db 100644
--- a/include/exec/cpu-all.h
+++ b/include/exec/cpu-all.h
@@ -189,6 +189,15 @@ void address_space_stq(AddressSpace *as, hwaddr addr, uint64_t val,
 
 /* page related stuff */
 
+#ifdef TARGET_PAGE_BITS_VARY
+extern bool target_page_bits_decided;
+extern int target_page_bits;
+#define TARGET_PAGE_BITS ({ assert(target_page_bits_decided); \
+                            target_page_bits; })
+#else
+#define TARGET_PAGE_BITS_MIN TARGET_PAGE_BITS
+#endif
+
 #define TARGET_PAGE_SIZE (1 << TARGET_PAGE_BITS)
 #define TARGET_PAGE_MASK ~(TARGET_PAGE_SIZE - 1)
 #define TARGET_PAGE_ALIGN(addr) (((addr) + TARGET_PAGE_SIZE - 1) & TARGET_PAGE_MASK)
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);
diff --git a/include/qemu-common.h b/include/qemu-common.h
index 9e8b0bd991..7e6e4feb4b 100644
--- a/include/qemu-common.h
+++ b/include/qemu-common.h
@@ -82,6 +82,18 @@ bool tcg_enabled(void);
 void cpu_exec_init_all(void);
 
 /**
+ * set_preferred_target_page_bits:
+ * @bits: number of bits needed to represent an address within the page
+ *
+ * Set the preferred target page size (the actual target page
+ * size may be smaller than any given CPU's preference).
+ * Returns true on success, false on failure (which can only happen
+ * if this is called after the system has already finalized its
+ * choice of page size and the requested page size is smaller than that).
+ */
+bool set_preferred_target_page_bits(int bits);
+
+/**
  * Sends a (part of) iovec down a socket, yielding when the socket is full, or
  * Receives data into a (part of) iovec from a socket,
  * yielding when there is no data in the socket.