Re: [Qemu-devel] [PATCH v3 6/6] Add dbus-vmstate object

[Daniel P . Berrangé]

On Thu, Sep 12, 2019 at 04:25:14PM +0400, Marc-André Lureau wrote:
> When instanciated, this object will connect to the given D-Bus
> bus. During migration, it will take the data from org.qemu.VMState1
> instances.
> 
> See documentation for further details.
> 
> Signed-off-by: Marc-André Lureau <address@hidden>
> ---
>  MAINTAINERS                   |   6 +
>  backends/Makefile.objs        |   4 +
>  backends/dbus-vmstate.c       | 530 ++++++++++++++++++++++++++++++++++
>  configure                     |   7 +
>  docs/interop/dbus-vmstate.rst |  68 +++++
>  docs/interop/index.rst        |   1 +
>  tests/Makefile.include        |  19 +-
>  tests/dbus-vmstate-daemon.sh  |  95 ++++++
>  tests/dbus-vmstate-test.c     | 386 +++++++++++++++++++++++++
>  tests/dbus-vmstate1.xml       |  12 +
>  10 files changed, 1127 insertions(+), 1 deletion(-)
>  create mode 100644 backends/dbus-vmstate.c
>  create mode 100644 docs/interop/dbus-vmstate.rst
>  create mode 100755 tests/dbus-vmstate-daemon.sh
>  create mode 100644 tests/dbus-vmstate-test.c
>  create mode 100644 tests/dbus-vmstate1.xml
> 
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 50eaf005f4..f641870c40 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -2153,6 +2153,12 @@ F: tests/migration-test.c
>  F: docs/devel/migration.rst
>  F: qapi/migration.json
>  
> +DBus VMState
> +M: Marc-André Lureau <address@hidden>
> +S: Maintained
> +F: backends/dbus-vmstate.c
> +F: tests/dbus-vmstate*
> +
>  Seccomp
>  M: Eduardo Otubo <address@hidden>
>  S: Supported
> diff --git a/backends/Makefile.objs b/backends/Makefile.objs
> index f0691116e8..28a847cd57 100644
> --- a/backends/Makefile.objs
> +++ b/backends/Makefile.objs
> @@ -17,3 +17,7 @@ endif
>  common-obj-$(call land,$(CONFIG_VHOST_USER),$(CONFIG_VIRTIO)) += vhost-user.o
>  
>  common-obj-$(CONFIG_LINUX) += hostmem-memfd.o
> +
> +common-obj-$(CONFIG_GIO) += dbus-vmstate.o
> +dbus-vmstate.o-cflags = $(GIO_CFLAGS)
> +dbus-vmstate.o-libs = $(GIO_LIBS)
> diff --git a/backends/dbus-vmstate.c b/backends/dbus-vmstate.c
> new file mode 100644
> index 0000000000..559f5430c5
> --- /dev/null
> +++ b/backends/dbus-vmstate.c
> @@ -0,0 +1,530 @@
> +/*
> + * QEMU dbus-vmstate
> + *
> + * Copyright (C) 2019 Red Hat Inc
> + *
> + * Authors:
> + *  Marc-André Lureau <address@hidden>
> + *
> + * This work is licensed under the terms of the GNU GPL, version 2 or later.
> + * See the COPYING file in the top-level directory.
> + */
> +
> +#include "qemu/osdep.h"
> +#include "qemu/units.h"
> +#include "qemu/error-report.h"
> +#include "qapi/error.h"
> +#include "qom/object_interfaces.h"
> +#include "qapi/qmp/qerror.h"
> +#include "migration/vmstate.h"
> +#include <gio/gio.h>
> +
> +typedef struct DBusVMState DBusVMState;
> +typedef struct DBusVMStateClass DBusVMStateClass;
> +
> +#define TYPE_DBUS_VMSTATE "dbus-vmstate"
> +#define DBUS_VMSTATE(obj)                                \
> +    OBJECT_CHECK(DBusVMState, (obj), TYPE_DBUS_VMSTATE)
> +#define DBUS_VMSTATE_GET_CLASS(obj)                              \
> +    OBJECT_GET_CLASS(DBusVMStateClass, (obj), TYPE_DBUS_VMSTATE)
> +#define DBUS_VMSTATE_CLASS(klass)                                    \
> +    OBJECT_CLASS_CHECK(DBusVMStateClass, (klass), TYPE_DBUS_VMSTATE)
> +
> +struct DBusVMStateClass {
> +    ObjectClass parent_class;
> +};
> +
> +struct DBusVMState {
> +    Object parent;
> +
> +    GDBusConnection *bus;
> +    char *dbus_addr;
> +    char *id_list;
> +
> +    uint32_t data_size;
> +    uint8_t *data;
> +};
> +
> +static const GDBusPropertyInfo vmstate_property_info[] = {
> +    { -1, (char *) "Id", (char *) "s",
> +      G_DBUS_PROPERTY_INFO_FLAGS_READABLE, NULL },
> +};
> +
> +static const GDBusPropertyInfo * const vmstate_property_info_pointers[] = {
> +    &vmstate_property_info[0],
> +    NULL
> +};
> +
> +static const GDBusInterfaceInfo vmstate1_interface_info = {
> +    -1,
> +    (char *) "org.qemu.VMState1",
> +    (GDBusMethodInfo **) NULL,
> +    (GDBusSignalInfo **) NULL,
> +    (GDBusPropertyInfo **) &vmstate_property_info_pointers,
> +    NULL,
> +};
> +
> +#define DBUS_VMSTATE_SIZE_LIMIT (1 * MiB)
> +
> +static char **
> +dbus_get_vmstate1_names(DBusVMState *self, GError **err)
> +{
> +    g_autoptr(GDBusProxy) proxy = NULL;
> +    g_autoptr(GVariant) result = NULL;
> +    g_autoptr(GVariant) child = NULL;
> +
> +    proxy = g_dbus_proxy_new_sync(self->bus, G_DBUS_PROXY_FLAGS_NONE, NULL,
> +                                  "org.freedesktop.DBus",
> +                                  "/org/freedesktop/DBus",
> +                                  "org.freedesktop.DBus",
> +                                  NULL, err);
> +    if (!proxy) {
> +        return NULL;
> +    }
> +
> +    result = g_dbus_proxy_call_sync(proxy, "ListQueuedOwners",
> +                                    g_variant_new("(s)", 
> "org.qemu.VMState1"),
> +                                    G_DBUS_CALL_FLAGS_NO_AUTO_START,
> +                                    -1, NULL, err);
> +    if (!result) {
> +        return NULL;
> +    }
> +
> +    child = g_variant_get_child_value(result, 0);
> +    return g_variant_dup_strv(child, NULL);
> +}

I'd probably move this into a util/dbus.c file, as it is the
kind of generic code that is likely to be useful in other
parts of QEMU. Just have "org.qem.VMState1" passed in as a
param.

> +
> +static GHashTable *
> +get_id_list_set(DBusVMState *self)
> +{
> +    g_auto(GStrv) ids = NULL;
> +    g_autoptr(GHashTable) set = NULL;
> +    int i;
> +
> +    if (!self->id_list) {
> +        return NULL;
> +    }
> +
> +    ids = g_strsplit(self->id_list, ",", -1);
> +    set = g_hash_table_new_full(g_str_hash, g_str_equal, g_free, NULL);
> +    for (i = 0; ids[i]; i++) {
> +        g_hash_table_add(set, ids[i]);
> +        ids[i] = NULL;
> +    }
> +
> +    return g_steal_pointer(&set);
> +}
> +
> +static GHashTable *
> +dbus_get_proxies(DBusVMState *self, GError **err)
> +{
> +    g_autoptr(GError) local_err = NULL;
> +    g_autoptr(GHashTable) proxies = NULL;
> +    g_autoptr(GHashTable) ids = NULL;
> +    g_auto(GStrv) names = NULL;
> +    size_t i;
> +
> +    ids = get_id_list_set(self);
> +    proxies = g_hash_table_new_full(g_str_hash, g_str_equal,
> +                                    g_free, g_object_unref);
> +
> +    names = dbus_get_vmstate1_names(self, &local_err);
> +    if (!names) {
> +        if (g_error_matches(local_err,
> +                            G_DBUS_ERROR, G_DBUS_ERROR_NAME_HAS_NO_OWNER)) {
> +            return proxies;
> +        }
> +        g_propagate_error(err, local_err);
> +        return NULL;
> +    }
> +
> +    for (i = 0; names[i]; i++) {
> +        g_autoptr(GDBusProxy) proxy = NULL;
> +        g_autoptr(GVariant) result = NULL;
> +        g_autofree char *id = NULL;
> +        size_t size;
> +
> +        proxy = g_dbus_proxy_new_sync(self->bus, G_DBUS_PROXY_FLAGS_NONE,
> +                    (GDBusInterfaceInfo *) &vmstate1_interface_info,
> +                    names[i],
> +                    "/org/qemu/VMState1",
> +                    "org.qemu.VMState1",
> +                    NULL, err);
> +        if (!proxy) {
> +            return NULL;
> +        }
> +
> +        result = g_dbus_proxy_get_cached_property(proxy, "Id");
> +        if (!result) {
> +            g_set_error_literal(err, G_IO_ERROR, G_IO_ERROR_FAILED,
> +                                "VMState Id property is missing.");
> +            return NULL;
> +        }
> +
> +        id = g_variant_dup_string(result, &size);
> +        if (ids && !g_hash_table_remove(ids, id)) {
> +            g_clear_pointer(&id, g_free);
> +            g_clear_object(&proxy);
> +            continue;
> +        }
> +        if (size == 0 || size >= 256) {
> +            g_set_error(err, G_IO_ERROR, G_IO_ERROR_FAILED,
> +                        "VMState Id '%s' is invalid.", id);
> +            return NULL;
> +        }
> +
> +        if (!g_hash_table_insert(proxies, id, proxy)) {
> +            g_set_error(err, G_IO_ERROR, G_IO_ERROR_FAILED,
> +                        "Duplicated VMState Id '%s'", id);
> +            return NULL;
> +        }
> +        id = NULL;
> +        proxy = NULL;
> +
> +        g_clear_pointer(&result, g_variant_unref);
> +    }
> +
> +    if (ids) {
> +        g_autofree char **left = NULL;
> +
> +        left = (char **)g_hash_table_get_keys_as_array(ids, NULL);
> +        if (*left) {
> +            g_autofree char *leftids = g_strjoinv(",", left);
> +            g_set_error(err, G_IO_ERROR, G_IO_ERROR_FAILED,
> +                        "Required VMState Id are missing: %s", leftids);
> +            return NULL;
> +        }
> +    }
> +
> +    return g_steal_pointer(&proxies);
> +}

This method could probably go to a util/dbus.c file too.

> +
> +static int
> +dbus_load_state_proxy(GDBusProxy *proxy, const uint8_t *data, size_t size)
> +{
> +    g_autoptr(GError) err = NULL;
> +    g_autoptr(GVariant) result = NULL;
> +    g_autoptr(GVariant) value = NULL;
> +
> +    value = g_variant_new_fixed_array(G_VARIANT_TYPE_BYTE,
> +                                      data, size, sizeof(char));
> +    result = g_dbus_proxy_call_sync(proxy, "Load",
> +                                    g_variant_new("(@ay)",
> +                                                  g_steal_pointer(&value)),
> +                                    G_DBUS_CALL_FLAGS_NO_AUTO_START,
> +                                    -1, NULL, &err);
> +    if (!result) {
> +        error_report("Failed to Load: %s", err->message);
> +        return -1;
> +    }
> +
> +    return 0;
> +}
> +
> +static int dbus_vmstate_post_load(void *opaque, int version_id)
> +{
> +    DBusVMState *self = DBUS_VMSTATE(opaque);
> +    g_autoptr(GInputStream) m = NULL;
> +    g_autoptr(GDataInputStream) s = NULL;
> +    g_autoptr(GError) err = NULL;
> +    g_autoptr(GHashTable) proxies = NULL;
> +    uint32_t nelem;
> +
> +    proxies = dbus_get_proxies(self, &err);
> +    if (!proxies) {
> +        error_report("Failed to get proxies: %s", err->message);
> +        return -1;
> +    }
> +
> +    m = g_memory_input_stream_new_from_data(self->data, self->data_size, 
> NULL);
> +    s = g_data_input_stream_new(m);
> +    g_data_input_stream_set_byte_order(s, 
> G_DATA_STREAM_BYTE_ORDER_BIG_ENDIAN);
> +
> +    nelem = g_data_input_stream_read_uint32(s, NULL, &err);
> +    if (err) {
> +        goto error;
> +    }
> +
> +    while (nelem > 0) {
> +        GDBusProxy *proxy = NULL;
> +        uint32_t len;
> +        gsize bytes_read, avail;
> +        char id[256];
> +
> +        len = g_data_input_stream_read_uint32(s, NULL, &err);
> +        if (err) {
> +            goto error;
> +        }
> +        if (len >= 256) {
> +            error_report("Invalid DBus vmstate proxy name %u", len);
> +            return -1;
> +        }
> +        if (!g_input_stream_read_all(G_INPUT_STREAM(s), id, len,
> +                                     &bytes_read, NULL, &err)) {
> +            goto error;
> +        }
> +        g_return_val_if_fail(bytes_read == len, -1);
> +        id[len] = 0;
> +
> +        proxy = g_hash_table_lookup(proxies, id);
> +        if (!proxy) {
> +            error_report("Failed to find proxy Id '%s'", id);
> +            return -1;
> +        }
> +
> +        len = g_data_input_stream_read_uint32(s, NULL, &err);
> +        avail = g_buffered_input_stream_get_available(
> +            G_BUFFERED_INPUT_STREAM(s));
> +
> +        if (len > DBUS_VMSTATE_SIZE_LIMIT || len > avail) {
> +            error_report("Invalid vmstate size: %u", len);
> +            return -1;
> +        }
> +
> +        if (dbus_load_state_proxy(proxy,
> +                
> g_buffered_input_stream_peek_buffer(G_BUFFERED_INPUT_STREAM(s),
> +                                                    NULL),
> +                len) < 0) {
> +            error_report("Failed to restore Id '%s'", id);
> +            return -1;
> +        }
> +
> +        if (!g_seekable_seek(G_SEEKABLE(s), len, G_SEEK_CUR, NULL, &err)) {
> +            goto error;
> +        }
> +
> +        nelem -= 1;
> +    }
> +
> +    return 0;
> +
> +error:
> +    error_report("Failed to read from stream: %s", err->message);
> +    return -1;
> +}
> +
> +static void
> +dbus_save_state_proxy(gpointer key,
> +                      gpointer value,
> +                      gpointer user_data)
> +{
> +    GDataOutputStream *s = user_data;
> +    const char *id = key;
> +    GDBusProxy *proxy = value;
> +    g_autoptr(GVariant) result = NULL;
> +    g_autoptr(GVariant) child = NULL;
> +    g_autoptr(GError) err = NULL;
> +    const uint8_t *data;
> +    gsize size;
> +
> +    result = g_dbus_proxy_call_sync(proxy, "Save",
> +                                    NULL, G_DBUS_CALL_FLAGS_NO_AUTO_START,
> +                                    -1, NULL, &err);
> +    if (!result) {
> +        error_report("Failed to Save: %s", err->message);
> +        return;
> +    }
> +
> +    child = g_variant_get_child_value(result, 0);
> +    data = g_variant_get_fixed_array(child, &size, sizeof(char));
> +    if (!data) {
> +        error_report("Failed to Save: not a byte array");
> +        return;
> +    }
> +    if (size > DBUS_VMSTATE_SIZE_LIMIT) {
> +        error_report("Too large vmstate data to save: %lu", size);
> +        return;
> +    }
> +
> +    if (!g_data_output_stream_put_uint32(s, strlen(id), NULL, &err) ||
> +        !g_data_output_stream_put_string(s, id, NULL, &err) ||
> +        !g_data_output_stream_put_uint32(s, size, NULL, &err) ||
> +        !g_output_stream_write_all(G_OUTPUT_STREAM(s),
> +                                   data, size, NULL, NULL, &err)) {
> +        error_report("Failed to write to stream: %s", err->message);
> +    }
> +}
> +
> +static int dbus_vmstate_pre_save(void *opaque)
> +{
> +    DBusVMState *self = DBUS_VMSTATE(opaque);
> +    g_autoptr(GOutputStream) m = NULL;
> +    g_autoptr(GDataOutputStream) s = NULL;
> +    g_autoptr(GHashTable) proxies = NULL;
> +    g_autoptr(GError) err = NULL;
> +
> +    proxies = dbus_get_proxies(self, &err);
> +    if (!proxies) {
> +        error_report("Failed to get proxies: %s", err->message);
> +        return -1;
> +    }
> +
> +    m = g_memory_output_stream_new_resizable();
> +    s = g_data_output_stream_new(m);
> +    g_data_output_stream_set_byte_order(s, 
> G_DATA_STREAM_BYTE_ORDER_BIG_ENDIAN);
> +
> +    if (!g_data_output_stream_put_uint32(s, g_hash_table_size(proxies),
> +                                         NULL, &err)) {
> +        error_report("Failed to write to stream: %s", err->message);
> +        return -1;
> +    }
> +
> +    g_hash_table_foreach(proxies, dbus_save_state_proxy, s);
> +
> +    if (g_memory_output_stream_get_size(G_MEMORY_OUTPUT_STREAM(m))
> +        > UINT32_MAX) {
> +        error_report("DBus vmstate buffer is too large");
> +        return -1;
> +    }
> +
> +    if (!g_output_stream_close(G_OUTPUT_STREAM(m), NULL, &err)) {
> +        error_report("Failed to close stream: %s", err->message);
> +        return -1;
> +    }
> +
> +    g_free(self->data);
> +    self->data_size =
> +        g_memory_output_stream_get_size(G_MEMORY_OUTPUT_STREAM(m));
> +    self->data =
> +        g_memory_output_stream_steal_data(G_MEMORY_OUTPUT_STREAM(m));
> +
> +    return 0;
> +}
> +
> +static const VMStateDescription dbus_vmstate = {
> +    .name = TYPE_DBUS_VMSTATE,
> +    .version_id = 0,
> +    .pre_save = dbus_vmstate_pre_save,
> +    .post_load = dbus_vmstate_post_load,
> +    .fields = (VMStateField[]) {
> +        VMSTATE_UINT32(data_size, DBusVMState),
> +        VMSTATE_VBUFFER_ALLOC_UINT32(data, DBusVMState, 0, 0, data_size),
> +        VMSTATE_END_OF_LIST()
> +    }
> +};
> +
> +static void
> +dbus_vmstate_complete(UserCreatable *uc, Error **errp)
> +{
> +    DBusVMState *self = DBUS_VMSTATE(uc);
> +    GError *err = NULL;
> +    GDBusConnection *bus;
> +
> +    if (!object_resolve_path_type("", TYPE_DBUS_VMSTATE, NULL)) {
> +        error_setg(errp, "There is already an instance of %s",
> +                   TYPE_DBUS_VMSTATE);
> +        return;
> +    }
> +
> +    if (!self->dbus_addr) {
> +        error_setg(errp, QERR_MISSING_PARAMETER, "addr");
> +        return;
> +    }
> +
> +    bus = g_dbus_connection_new_for_address_sync(self->dbus_addr,
> +             G_DBUS_CONNECTION_FLAGS_AUTHENTICATION_CLIENT |
> +             G_DBUS_CONNECTION_FLAGS_MESSAGE_BUS_CONNECTION,
> +             NULL, NULL, &err);
> +    if (err) {
> +        error_setg(errp, "failed to connect to DBus: '%s'", err->message);
> +        g_clear_error(&err);
> +    }
> +
> +    self->bus = bus;
> +
> +    if (vmstate_register(VMSTATE_IF(self), -1, &dbus_vmstate, self) < 0) {
> +        error_setg(errp, "Failed to register vmstate");
> +    }
> +}
> +
> +static void
> +dbus_vmstate_finalize(Object *o)
> +{
> +    DBusVMState *self = DBUS_VMSTATE(o);
> +
> +    vmstate_unregister(VMSTATE_IF(self), &dbus_vmstate, self);
> +
> +    g_clear_object(&self->bus);
> +    g_free(self->dbus_addr);
> +    g_free(self->id_list);
> +    g_free(self->data);
> +}
> +
> +static char *
> +get_dbus_addr(Object *o, Error **errp)
> +{
> +    DBusVMState *self = DBUS_VMSTATE(o);
> +
> +    return g_strdup(self->dbus_addr);
> +}
> +
> +static void
> +set_dbus_addr(Object *o, const char *str, Error **errp)
> +{
> +    DBusVMState *self = DBUS_VMSTATE(o);
> +
> +    g_free(self->dbus_addr);
> +    self->dbus_addr = g_strdup(str);
> +}
> +
> +static char *
> +get_id_list(Object *o, Error **errp)
> +{
> +    DBusVMState *self = DBUS_VMSTATE(o);
> +
> +    return g_strdup(self->id_list);
> +}
> +
> +static void
> +set_id_list(Object *o, const char *str, Error **errp)
> +{
> +    DBusVMState *self = DBUS_VMSTATE(o);
> +
> +    g_free(self->id_list);
> +    self->id_list = g_strdup(str);
> +}
> +
> +static char *
> +dbus_vmstate_get_id(VMStateIf *vmif)
> +{
> +    return g_strdup(TYPE_DBUS_VMSTATE);
> +}
> +
> +static void
> +dbus_vmstate_class_init(ObjectClass *oc, void *data)
> +{
> +    UserCreatableClass *ucc = USER_CREATABLE_CLASS(oc);
> +    VMStateIfClass *vc = VMSTATE_IF_CLASS(oc);
> +
> +    ucc->complete = dbus_vmstate_complete;
> +    vc->get_id = dbus_vmstate_get_id;
> +
> +    object_class_property_add_str(oc, "addr",
> +                                  get_dbus_addr, set_dbus_addr,
> +                                  &error_abort);
> +    object_class_property_add_str(oc, "id-list",
> +                                  get_id_list, set_id_list,
> +                                  &error_abort);
> +}
> +
> +static const TypeInfo dbus_vmstate_info = {
> +    .name = TYPE_DBUS_VMSTATE,
> +    .parent = TYPE_OBJECT,
> +    .instance_size = sizeof(DBusVMState),
> +    .instance_finalize = dbus_vmstate_finalize,
> +    .class_size = sizeof(DBusVMStateClass),
> +    .class_init = dbus_vmstate_class_init,
> +    .interfaces = (InterfaceInfo[]) {
> +        { TYPE_USER_CREATABLE },
> +        { TYPE_VMSTATE_IF },
> +        { }
> +    }
> +};
> +
> +static void
> +register_types(void)
> +{
> +    type_register_static(&dbus_vmstate_info);
> +}
> +
> +type_init(register_types);
> diff --git a/configure b/configure
> index 95134c0180..0a6eb2ebcd 100755
> --- a/configure
> +++ b/configure
> @@ -3665,10 +3665,16 @@ if $pkg_config --atleast-version=$glib_req_ver 
> gio-2.0; then
>      gio=yes
>      gio_cflags=$($pkg_config --cflags gio-2.0)
>      gio_libs=$($pkg_config --libs gio-2.0)
> +    gdbus_codegen=$($pkg_config --variable=gdbus_codegen gio-2.0)
>  else
>      gio=no
>  fi
>  
> +if $pkg_config --atleast-version=$glib_req_ver gio-unix-2.0; then
> +    gio_cflags="$gio_cflags $($pkg_config --cflags gio-unix-2.0)"
> +    gio_libs="$gio_libs $($pkg_config --libs gio-unix-2.0)"
> +fi

Unless I'm missing it, nothing in this patch uses gio-unix APIs


> diff --git a/docs/interop/dbus-vmstate.rst b/docs/interop/dbus-vmstate.rst
> new file mode 100644
> index 0000000000..78070be1bd
> --- /dev/null
> +++ b/docs/interop/dbus-vmstate.rst
> @@ -0,0 +1,68 @@
> +=============
> +D-Bus VMState
> +=============
> +
> +Introduction
> +============
> +
> +The QEMU dbus-vmstate object aim is to migrate helpers data running on
> +a QEMU D-Bus bus. (refer to the :doc:`dbus` document for
> +recommendation on D-Bus usage)
> +
> +Upon migration, QEMU will go through the queue of
> +``org.qemu.VMState1`` D-Bus name owners and query their ``Id``. It
> +must be unique among the helpers.
> +
> +It will then save arbitrary data of each Id to be transferred in the
> +migration stream and restored/loaded at the corresponding destination
> +helper.
> +
> +The data amount to be transferred is limited to 1Mb. The state must be
> +saved quickly (a few seconds maximum). (D-Bus imposes a time limit on
> +reply anyway, and migration would fail if data isn't given quickly
> +enough)
> +
> +dbus-vmstate object can be configured with the expected list of
> +helpers by setting its ``id-list`` property, with a coma-separated
> +``Id`` list.
> +
> +Interface
> +=========
> +
> +On object path ``/org/qemu/VMState1``, the following
> +``org.qemu.VMState1`` interface should be implemented:
> +
> +.. code:: xml
> +
> +  <interface name="org.qemu.VMState1">
> +    <property name="Id" type="s" access="read"/>
> +    <method name="Load">
> +      <arg type="ay" name="data" direction="in"/>
> +    </method>
> +    <method name="Save">
> +      <arg type="ay" name="data" direction="out"/>
> +    </method>
> +  </interface>
> +
> +"Id" property
> +-------------
> +
> +A string that identifies the helper uniquely.

Is there any association to the "id" values used in QEMU
-object/-device objects, or is this a separate ID namespace
entirely ?

> +Load(in u8[] bytes) method
> +--------------------------
> +
> +The method called on destination with the state to restore.
> +
> +The helper may be initially started in a waiting state (with
> +an --incoming argument for example), and it may resume on success.
> +
> +An error may be returned to the caller.
> +
> +Save(out u8[] bytes) method
> +---------------------------
> +
> +The method called on the source to get the current state to be
> +migrated. The helper should continue to run normally.
> +
> +An error may be returned to the caller.
> diff --git a/docs/interop/index.rst b/docs/interop/index.rst
> index fa4478ce2e..75832f44ac 100644
> --- a/docs/interop/index.rst
> +++ b/docs/interop/index.rst
> @@ -14,6 +14,7 @@ Contents:
>  
>     bitmaps
>     dbus
> +   dbus-vmstate
>     live-block-operations
>     pr-helper
>     vhost-user

Perhaps the 'dbus.rst' doc should have a link to the
'dbus-vmstate' block under a "QEMU services/interfaces" heading


Regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|