[PATCH v6 16/19] softmmu/vl: defer backend init

[Jagannathan Raman]

Allow deferred initialization of backends. TYPE_REMOTE_MACHINE is
agnostic to QEMU's RUN_STATE. It's state is driven by the QEMU client
via the vfio-user protocol. Whereas, the backends presently defer
initialization if QEMU is in RUN_STATE_INMIGRATE. Since the remote
machine can't use RUN_STATE*, this commit allows it to ask for deferred
initialization of backend device. It is primarily targeted towards block
devices in this commit, but it needed not be limited to that.

Signed-off-by: Elena Ufimtseva <elena.ufimtseva@oracle.com>
Signed-off-by: John G Johnson <john.g.johnson@oracle.com>
Signed-off-by: Jagannathan Raman <jag.raman@oracle.com>
---
 include/sysemu/sysemu.h    |  4 ++++
 block/block-backend.c      |  3 ++-
 blockdev.c                 |  2 +-
 softmmu/vl.c               | 17 +++++++++++++++++
 stubs/defer-backend-init.c |  7 +++++++
 MAINTAINERS                |  1 +
 stubs/meson.build          |  1 +
 7 files changed, 33 insertions(+), 2 deletions(-)
 create mode 100644 stubs/defer-backend-init.c

diff --git a/include/sysemu/sysemu.h b/include/sysemu/sysemu.h
index b9421e03ff..3179eb1857 100644
--- a/include/sysemu/sysemu.h
+++ b/include/sysemu/sysemu.h
@@ -119,4 +119,8 @@ extern QemuOptsList qemu_net_opts;
 extern QemuOptsList qemu_global_opts;
 extern QemuOptsList qemu_semihosting_config_opts;
 
+bool deferred_backend_init(void);
+void set_deferred_backend_init(void);
+void clear_deferred_backend_init(void);
+
 #endif
diff --git a/block/block-backend.c b/block/block-backend.c
index 4ff6b4d785..e04f9b6469 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -20,6 +20,7 @@
 #include "sysemu/blockdev.h"
 #include "sysemu/runstate.h"
 #include "sysemu/replay.h"
+#include "sysemu/sysemu.h"
 #include "qapi/error.h"
 #include "qapi/qapi-events-block.h"
 #include "qemu/id.h"
@@ -935,7 +936,7 @@ int blk_attach_dev(BlockBackend *blk, DeviceState *dev)
     /* While migration is still incoming, we don't need to apply the
      * permissions of guest device BlockBackends. We might still have a block
      * job or NBD server writing to the image for storage migration. */
-    if (runstate_check(RUN_STATE_INMIGRATE)) {
+    if (runstate_check(RUN_STATE_INMIGRATE) || deferred_backend_init()) {
         blk->disable_perm = true;
     }
 
diff --git a/blockdev.c b/blockdev.c
index 42e098b458..d495070679 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -569,7 +569,7 @@ static BlockBackend *blockdev_init(const char *file, QDict 
*bs_opts,
         qdict_set_default_str(bs_opts, BDRV_OPT_AUTO_READ_ONLY, "on");
         assert((bdrv_flags & BDRV_O_CACHE_MASK) == 0);
 
-        if (runstate_check(RUN_STATE_INMIGRATE)) {
+        if (runstate_check(RUN_STATE_INMIGRATE) || deferred_backend_init()) {
             bdrv_flags |= BDRV_O_INACTIVE;
         }
 
diff --git a/softmmu/vl.c b/softmmu/vl.c
index 5e1b35ba48..9584ab82e3 100644
--- a/softmmu/vl.c
+++ b/softmmu/vl.c
@@ -496,6 +496,23 @@ static QemuOptsList qemu_action_opts = {
     },
 };
 
+bool defer_backend_init;
+
+bool deferred_backend_init(void)
+{
+    return defer_backend_init;
+}
+
+void set_deferred_backend_init(void)
+{
+    defer_backend_init = true;
+}
+
+void clear_deferred_backend_init(void)
+{
+    defer_backend_init = false;
+}
+
 const char *qemu_get_vm_name(void)
 {
     return qemu_name;
diff --git a/stubs/defer-backend-init.c b/stubs/defer-backend-init.c
new file mode 100644
index 0000000000..3a74c669a1
--- /dev/null
+++ b/stubs/defer-backend-init.c
@@ -0,0 +1,7 @@
+#include "qemu/osdep.h"
+#include "sysemu/sysemu.h"
+
+bool deferred_backend_init(void)
+{
+    return false;
+}
diff --git a/MAINTAINERS b/MAINTAINERS
index e274cb46af..1f55d04ce6 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -3572,6 +3572,7 @@ F: hw/remote/vfio-user-obj.c
 F: include/hw/remote/vfio-user-obj.h
 F: hw/remote/iommu.c
 F: include/hw/remote/iommu.h
+F: stubs/defer-backend-init.c
 
 EBPF:
 M: Jason Wang <jasowang@redhat.com>
diff --git a/stubs/meson.build b/stubs/meson.build
index c5ce979dc3..98770966f6 100644
--- a/stubs/meson.build
+++ b/stubs/meson.build
@@ -58,3 +58,4 @@ else
   stub_ss.add(files('qdev.c'))
 endif
 stub_ss.add(when: 'CONFIG_VFIO_USER_SERVER', if_false: 
files('vfio-user-obj.c'))
+stub_ss.add(files('defer-backend-init.c'))
-- 
2.20.1