In QEMU, a machine and the main_system_bus always go togehter. Usually
the bus is part of the machine which suggsts to host it there.

Since tere is already a current_machine singleton, all code that
accesses the main_system_bus can be changed (behind the scenes) to go
through current_machine. This resolves a singleton. Futhermore, by
reifying it in code, the every-machine-has-exactly-one-main-system-bus
relationship becomes very obvious.

Note that the main_system_bus attribute is a value rather than a
pointer. This trades pointer dereferences for pointer arithmetic. The
idea is to reduce cache misses - a rule of thumb says that
every pointer dereference causes a cache miss while arithmetic is
basically free.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
---
 hw/core/bus.c       |  5 ++++-
 hw/core/machine.c   |  3 +++
 hw/core/sysbus.c    | 22 +++++-----------------
 include/hw/boards.h |  1 +
 4 files changed, 13 insertions(+), 18 deletions(-)

diff --git a/hw/core/bus.c b/hw/core/bus.c
index c7831b5293..e3e807946c 100644
--- a/hw/core/bus.c
+++ b/hw/core/bus.c
@@ -129,9 +129,12 @@ static void qbus_init_internal(BusState *bus, DeviceState *parent,
         bus->parent->num_child_bus++;
         object_property_add_child(OBJECT(bus->parent), bus->name, OBJECT(bus));
         object_unref(OBJECT(bus));
+
+        /* The only bus without a parent is the main system bus */
+        assert(sysbus_get_default());
     } else {
         /* The only bus without a parent is the main system bus */
-        assert(bus == sysbus_get_default());
+        assert(!sysbus_get_default());
     }
 }
 
diff --git a/hw/core/machine.c b/hw/core/machine.c
index aa520e74a8..ebd3e0ff08 100644
--- a/hw/core/machine.c
+++ b/hw/core/machine.c
@@ -1097,6 +1097,9 @@ static void machine_initfn(Object *obj)
     ms->smp.threads = 1;
 
     machine_copy_boot_config(ms, &(BootConfiguration){ 0 });
+
+    qbus_init(&ms->main_system_bus, sizeof(ms->main_system_bus),
+              TYPE_SYSTEM_BUS, NULL, "main-system-bus");
 }
 
 static void machine_finalize(Object *obj)
diff --git a/hw/core/sysbus.c b/hw/core/sysbus.c
index 05c1da3d31..16a9b4d7a0 100644
--- a/hw/core/sysbus.c
+++ b/hw/core/sysbus.c
@@ -20,6 +20,7 @@
 #include "qemu/osdep.h"
 #include "qapi/error.h"
 #include "qemu/module.h"
+#include "hw/boards.h"
 #include "hw/sysbus.h"
 #include "monitor/monitor.h"
 #include "exec/address-spaces.h"
@@ -336,26 +337,13 @@ static const TypeInfo sysbus_device_type_info = {
     .class_init = sysbus_device_class_init,
 };
 
-static BusState *main_system_bus;
-
-static void main_system_bus_create(void)
-{
-    /*
-     * assign main_system_bus before qbus_init()
-     * in order to make "if (bus != sysbus_get_default())" work
-     */
-    main_system_bus = g_malloc0(system_bus_info.instance_size);
-    qbus_init(main_system_bus, system_bus_info.instance_size,
-              TYPE_SYSTEM_BUS, NULL, "main-system-bus");
-    OBJECT(main_system_bus)->free = g_free;
-}
-
 BusState *sysbus_get_default(void)
 {
-    if (!main_system_bus) {
-        main_system_bus_create();
+    if (!current_machine) {
+        return NULL;
     }
-    return main_system_bus;
+
+    return &current_machine->main_system_bus;
 }
 
 static void sysbus_register_types(void)
diff --git a/include/hw/boards.h b/include/hw/boards.h
index 311ed17e18..7af940102d 100644
--- a/include/hw/boards.h
+++ b/include/hw/boards.h
@@ -346,6 +346,7 @@ struct MachineState {
      */
     MemoryRegion *ram;
     DeviceMemoryState *device_memory;
+    BusState main_system_bus;
 
     ram_addr_t ram_size;
     ram_addr_t maxram_size;
-- 
2.37.3

On 20/9/22 01:17, Bernhard Beschow wrote:
> In QEMU, a machine and the main_system_bus always go togehter. Usually
> the bus is part of the machine which suggsts to host it there.

"together", "suggests"

> Since tere is already a current_machine singleton, all code that
> accesses the main_system_bus can be changed (behind the scenes) to go
> through current_machine. This resolves a singleton. Futhermore, by

"Furthermore"

> reifying it in code, the every-machine-has-exactly-one-main-system-bus
> relationship becomes very obvious.
> 
> Note that the main_system_bus attribute is a value rather than a
> pointer. This trades pointer dereferences for pointer arithmetic. The
> idea is to reduce cache misses - a rule of thumb says that
> every pointer dereference causes a cache miss while arithmetic is
> basically free.
> 
> Signed-off-by: Bernhard Beschow <shentey@gmail.com>
> ---
>   hw/core/bus.c       |  5 ++++-
>   hw/core/machine.c   |  3 +++
>   hw/core/sysbus.c    | 22 +++++-----------------
>   include/hw/boards.h |  1 +
>   4 files changed, 13 insertions(+), 18 deletions(-)

> diff --git a/include/hw/boards.h b/include/hw/boards.h
> index 311ed17e18..7af940102d 100644
> --- a/include/hw/boards.h
> +++ b/include/hw/boards.h

Likely missing the BusState declaration:

   #include "hw/qdev-core.h"

> @@ -346,6 +346,7 @@ struct MachineState {
>        */
>       MemoryRegion *ram;
>       DeviceMemoryState *device_memory;
> +    BusState main_system_bus;
>   
>       ram_addr_t ram_size;
>       ram_addr_t maxram_size;