fw-cfg: bump "x-file-slots" to 0x20 for 2.9+ machine types

More precisely, the "x-file-slots" count is bumped for all machine types
that:
(a) use fw_cfg, and
(b) are not versioned (hence migration is not expected to work for them
    across QEMU releases anyway), or have version 2.9.

This affects machine types implemented in the following source files:

- "hw/arm/virt.c". The "virt-*" machine type is versioned, and the <= 2.8
  versions already depend on HW_COMPAT_2_8 (see commit e353aac51b).
  Therefore adding the "x-file-slots" compat values to HW_COMPAT_2_8
  suffices.

- "hw/i386/pc.c". The "pc-i440fx-*" (including "pc-*") and "pc-q35-*"
  machine types are versioned. Modifying HW_COMPAT_2_8 is sufficient here
  too (see commit "pc: Add 2.9 machine-types"). The "isapc" machtype is
  not versioned. The "xenfv" machine type, which uses fw_cfg for direct
  kernel booting, is also not versioned.

- "hw/ppc/mac_newworld.c". The "mac99" machine type is not versioned.

- "hw/ppc/mac_oldworld.c". The "g3beige" machine type is not versioned.

- "hw/sparc/sun4m.c". None of the 9 machine types defined in this file
  appear versioned.

- "hw/sparc64/sun4u.c". None of the 3 machine types defined in this file
  appear versioned.

Cc: "Gabriel L. Somlo" <somlo@cmu.edu>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Alexander Graf <agraf@suse.de>
Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Artyom Tarasenko <atar4qemu@gmail.com>
Cc: David Gibson <david@gibson.dropbear.id.au>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Igor Mammedov <imammedo@redhat.com>
Cc: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Acked-by: Gabriel Somlo <somlo@cmu.edu>
Tested-by: Gabriel Somlo <somlo@cmu.edu>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>

docs/specs/fw_cfg.txt

@@ -155,8 +155,8 @@ Selector Reg.    Range Usage
                                  through the DMA interface in QEMU v2.9+)
 0xc000 - 0xffff  Arch. Specific (0x0000 - 0x3fff, RW, ignored in v2.4+)
 
-In practice, the number of allowed firmware configuration items is given
-by the value (FW_CFG_FILE_FIRST + FW_CFG_FILE_SLOTS_MIN) (see fw_cfg.h).
+In practice, the number of allowed firmware configuration items depends on the
+machine type/version.
 
 = Guest-side DMA Interface =
 

hw/nvram/fw_cfg.c

@@ -35,6 +35,8 @@
 #include "qemu/cutils.h"
 #include "qapi/error.h"
 
+#define FW_CFG_FILE_SLOTS_DFLT 0x20
+
 #define FW_CFG_NAME "fw_cfg"
 #define FW_CFG_PATH "/machine/" FW_CFG_NAME
 
@@ -1058,7 +1060,7 @@ static Property fw_cfg_io_properties[] = {
     DEFINE_PROP_BOOL("dma_enabled", FWCfgIoState, parent_obj.dma_enabled,
                      true),
     DEFINE_PROP_UINT16("x-file-slots", FWCfgIoState, parent_obj.file_slots,
-                       FW_CFG_FILE_SLOTS_MIN),
+                       FW_CFG_FILE_SLOTS_DFLT),
     DEFINE_PROP_END_OF_LIST(),
 };
 
@@ -1110,7 +1112,7 @@ static Property fw_cfg_mem_properties[] = {
     DEFINE_PROP_BOOL("dma_enabled", FWCfgMemState, parent_obj.dma_enabled,
                      true),
     DEFINE_PROP_UINT16("x-file-slots", FWCfgMemState, parent_obj.file_slots,
-                       FW_CFG_FILE_SLOTS_MIN),
+                       FW_CFG_FILE_SLOTS_DFLT),
     DEFINE_PROP_END_OF_LIST(),
 };
 

include/hw/compat.h

@@ -2,7 +2,15 @@
 #define HW_COMPAT_H
 
 #define HW_COMPAT_2_8 \
-    /* empty */
+    {\
+        .driver   = "fw_cfg_mem",\
+        .property = "x-file-slots",\
+        .value    = stringify(0x10),\
+    },{\
+        .driver   = "fw_cfg_io",\
+        .property = "x-file-slots",\
+        .value    = stringify(0x10),\
+    },
 
 #define HW_COMPAT_2_7 \
     {\