[PATCH v3 15/28] hw/xen: do not repeatedly try to create a failing backend device

David Woodhouse
2023-10-25T14:52:09Z

From: David Woodhouse <dwmw@amazon.co.uk>

If xen_backend_device_create() fails to instantiate a device, the XenBus
code will just keep trying over and over again each time the bus is
re-enumerated, as long as the backend appears online and in
XenbusStateInitialising.

The only thing which prevents the XenBus code from recreating duplicates
of devices which already exist, is the fact that xen_device_realize()
sets the backend state to XenbusStateInitWait. If the attempt to create
the device doesn't get *that* far, that's when it will keep getting
retried.

My first thought was to handle errors by setting the backend state to
XenbusStateClosed, but that doesn't work for XenConsole which wants to
*ignore* any device of type != "ioemu" completely.

So, make xen_backend_device_create() *keep* the XenBackendInstance for a
failed device, and provide a new xen_backend_exists() function to allow
xen_bus_type_enumerate() to check whether one already exists before
creating a new one.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>
---
hw/xen/xen-backend.c | 27 +++++++++++++++++++++------
hw/xen/xen-bus.c | 3 ++-
include/hw/xen/xen-backend.h | 1 +
3 files changed, 24 insertions(+), 7 deletions(-)

diff --git a/hw/xen/xen-backend.c b/hw/xen/xen-backend.c
index 5b0fb76eae..b9bf70a9f5 100644
--- a/hw/xen/xen-backend.c
+++ b/hw/xen/xen-backend.c
@@ -101,6 +101,24 @@ static XenBackendInstance *xen_backend_list_find(XenDevice *xendev)
return NULL;
}

+bool xen_backend_exists(const char *type, const char *name)
+{
+ const XenBackendImpl *impl = xen_backend_table_lookup(type);
+ XenBackendInstance *backend;
+
+ if (!impl) {
+ return false;
+ }
+
+ QLIST_FOREACH(backend, &backend_list, entry) {
+ if (backend->impl == impl && !strcmp(backend->name, name)) {
+ return true;
+ }
+ }
+
+ return false;
+}
+
static void xen_backend_list_remove(XenBackendInstance *backend)
{
QLIST_REMOVE(backend, entry);
@@ -122,11 +140,6 @@ void xen_backend_device_create(XenBus *xenbus, const char *type,
backend->name = g_strdup(name);

impl->create(backend, opts, errp);
- if (*errp) {
- g_free(backend->name);
- g_free(backend);
- return;
- }

backend->impl = impl;
xen_backend_list_add(backend);
@@ -165,7 +178,9 @@ bool xen_backend_try_device_destroy(XenDevice *xendev, Error **errp)
}

impl = backend->impl;
- impl->destroy(backend, errp);
+ if (backend->xendev) {
+ impl->destroy(backend, errp);
+ }

xen_backend_list_remove(backend);
g_free(backend->name);
diff --git a/hw/xen/xen-bus.c b/hw/xen/xen-bus.c
index 12ff782005..3ffd1a5333 100644
--- a/hw/xen/xen-bus.c
+++ b/hw/xen/xen-bus.c
@@ -209,7 +209,8 @@ static void xen_bus_type_enumerate(XenBus *xenbus, const char *type)
NULL, "%u", &online) != 1)
online = 0;

- if (online && state == XenbusStateInitialising) {
+ if (online && state == XenbusStateInitialising &&
+ !xen_backend_exists(type, backend[i])) {
Error *local_err = NULL;

xen_bus_backend_create(xenbus, type, backend[i], backend_path,
diff --git a/include/hw/xen/xen-backend.h b/include/hw/xen/xen-backend.h
index aac2fd454d..0f01631ae7 100644
--- a/include/hw/xen/xen-backend.h
+++ b/include/hw/xen/xen-backend.h
@@ -33,6 +33,7 @@ XenDevice *xen_backend_get_device(XenBackendInstance *backend);
void xen_backend_register(const XenBackendInfo *info);
const char **xen_backend_get_types(unsigned int *nr);

+bool xen_backend_exists(const char *type, const char *name);
void xen_backend_device_create(XenBus *xenbus, const char *type,
const char *name, QDict *opts, Error **errp);
bool xen_backend_try_device_destroy(XenDevice *xendev, Error **errp);
--
2.40.1