cpu: introduce CPUClass::virtio_is_big_endian()

If we want to support targets that can change endianness (modern PPC and
ARM for the moment), we need to add a per-CPU class method to be called
from the virtio code. The virtio_ prefix in the name is a hint for people
to avoid misusage (aka. anywhere but from the virtio code).

The default behaviour is to return the compile-time default target
endianness.
Suggested-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NGreg Kurz <gkurz@linux.vnet.ibm.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

include/qom/cpu.h

@@ -116,6 +116,7 @@ typedef struct CPUClass {
     CPUUnassignedAccess do_unassigned_access;
     void (*do_unaligned_access)(CPUState *cpu, vaddr addr,
                                 int is_write, int is_user, uintptr_t retaddr);
+    bool (*virtio_is_big_endian)(CPUState *cpu);
     int (*memory_rw_debug)(CPUState *cpu, vaddr addr,
                            uint8_t *buf, int len, bool is_write);
     void (*dump_state)(CPUState *cpu, FILE *f, fprintf_function cpu_fprintf,

qom/cpu.c

@@ -196,6 +196,11 @@ static int cpu_common_gdb_write_register(CPUState *cpu, uint8_t *buf, int reg)
     return 0;
 }
 
+bool target_words_bigendian(void);
+static bool cpu_common_virtio_is_big_endian(CPUState *cpu)
+{
+    return target_words_bigendian();
+}
 
 void cpu_dump_state(CPUState *cpu, FILE *f, fprintf_function cpu_fprintf,
                     int flags)
@@ -334,6 +339,7 @@ static void cpu_class_init(ObjectClass *klass, void *data)
     k->write_elf64_note = cpu_common_write_elf64_note;
     k->gdb_read_register = cpu_common_gdb_read_register;
     k->gdb_write_register = cpu_common_gdb_write_register;
+    k->virtio_is_big_endian = cpu_common_virtio_is_big_endian;
     dc->realize = cpu_common_realizefn;
     /*
      * Reason: CPUs still need special care by board code: wiring up