pc/fwcfg: unbreak migration from qemu-2.5 and qemu-2.6 during firmware boot

Since 2.7 commit (b2a575a Add optionrom compatible with fw_cfg DMA version)
regressed migration during firmware exection time by
abusing fwcfg.dma_enabled property to decide loading
dma version of option rom AND by mistake disabling DMA
for 2.6 and earlier globally instead of only for option rom.

so 2.6 machine type guest is broken when it already runs
firmware in DMA mode but migrated to qemu-2.7(pc-2.6)
at that time;

a) qemu-2.6:pc2.6 (fwcfg.dma=on,firmware=dma,oprom=ioport)
b) qemu-2.7:pc2.6 (fwcfg.dma=off,firmware=ioport,oprom=ioport)

  to:   a     b
from
a       OK   FAIL
b       OK   OK

So we currently have broken forward migration from
qemu-2.6 to qemu-2.[789] that however could be fixed
for 2.10 by re-enabling DMA for 2.[56] machine types
and allowing dma capable option rom only since 2.7.
As result qemu should end up with:

c) qemu-2.10:pc2.6 (fwcfg.dma=on,firmware=dma,oprom=ioport)

   to:  a     b    c
from
a      OK   FAIL  OK
b      OK   OK    OK
c      OK   FAIL  OK

where forward migration from qemu-2.6 to qemu-2.10 should
work again leaving only qemu-2.[789]:pc-2.6 broken.

Reported-by: Eduardo Habkost <ehabkost@redhat.com>
Analyzed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

3 files changed, 6 insertions, 5 deletions

diff --git a/hw/i386/pc.c b/hw/i386/pc.c
index f3b372a..8063241 100644
--- a/hw/i386/pc.c
+++ b/hw/i386/pc.c
@@ -1047,12 +1047,10 @@ static void load_linux(PCMachineState *pcms,
     fw_cfg_add_i32(fw_cfg, FW_CFG_SETUP_SIZE, setup_size);
     fw_cfg_add_bytes(fw_cfg, FW_CFG_SETUP_DATA, setup, setup_size);
 
-    if (fw_cfg_dma_enabled(fw_cfg)) {
+    option_rom[nb_option_roms].bootindex = 0;
+    option_rom[nb_option_roms].name = "linuxboot.bin";
+    if (pcmc->linuxboot_dma_enabled && fw_cfg_dma_enabled(fw_cfg)) {
         option_rom[nb_option_roms].name = "linuxboot_dma.bin";
-        option_rom[nb_option_roms].bootindex = 0;
-    } else {
-        option_rom[nb_option_roms].name = "linuxboot.bin";
-        option_rom[nb_option_roms].bootindex = 0;
     }
     nb_option_roms++;
 }
@@ -2321,6 +2319,7 @@ static void pc_machine_class_init(ObjectClass *oc, void *data)
      * to be used at the moment, 32K should be enough for a while.  */
     pcmc->acpi_data_size = 0x20000 + 0x8000;
     pcmc->save_tsc_khz = true;
+    pcmc->linuxboot_dma_enabled = true;
     mc->get_hotplug_handler = pc_get_hotpug_handler;
     mc->cpu_index_to_socket_id = pc_cpu_index_to_socket_id;
     mc->possible_cpu_arch_ids = pc_possible_cpu_arch_ids;
diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
index 9f102aa..a11190b 100644
--- a/hw/i386/pc_piix.c
+++ b/hw/i386/pc_piix.c
@@ -474,6 +474,7 @@ static void pc_i440fx_2_6_machine_options(MachineClass *m)
     PCMachineClass *pcmc = PC_MACHINE_CLASS(m);
     pc_i440fx_2_7_machine_options(m);
     pcmc->legacy_cpu_hotplug = true;
+    pcmc->linuxboot_dma_enabled = false;
     SET_MACHINE_COMPAT(m, PC_COMPAT_2_6);
 }
 
diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
index dd792a8..0a61a20 100644
--- a/hw/i386/pc_q35.c
+++ b/hw/i386/pc_q35.c
@@ -335,6 +335,7 @@ static void pc_q35_2_6_machine_options(MachineClass *m)
     PCMachineClass *pcmc = PC_MACHINE_CLASS(m);
     pc_q35_2_7_machine_options(m);
     pcmc->legacy_cpu_hotplug = true;
+    pcmc->linuxboot_dma_enabled = false;
     SET_MACHINE_COMPAT(m, PC_COMPAT_2_6);
 }