xemu/blockdev-nbd.c
Eric Blake 58a6fdcc9e nbd/server: Allow MULTI_CONN for shared writable exports
According to the NBD spec, a server that advertises
NBD_FLAG_CAN_MULTI_CONN promises that multiple client connections will
not see any cache inconsistencies: when properly separated by a single
flush, actions performed by one client will be visible to another
client, regardless of which client did the flush.

We always satisfy these conditions in qemu - even when we support
multiple clients, ALL clients go through a single point of reference
into the block layer, with no local caching.  The effect of one client
is instantly visible to the next client.  Even if our backend were a
network device, we argue that any multi-path caching effects that
would cause inconsistencies in back-to-back actions not seeing the
effect of previous actions would be a bug in that backend, and not the
fault of caching in qemu.  As such, it is safe to unconditionally
advertise CAN_MULTI_CONN for any qemu NBD server situation that
supports parallel clients.

Note, however, that we don't want to advertise CAN_MULTI_CONN when we
know that a second client cannot connect (for historical reasons,
qemu-nbd defaults to a single connection while nbd-server-add and QMP
commands default to unlimited connections; but we already have
existing means to let either style of NBD server creation alter those
defaults).  This is visible by no longer advertising MULTI_CONN for
'qemu-nbd -r' without -e, as in the iotest nbd-qemu-allocation.

The harder part of this patch is setting up an iotest to demonstrate
behavior of multiple NBD clients to a single server.  It might be
possible with parallel qemu-io processes, but I found it easier to do
in python with the help of libnbd, and help from Nir and Vladimir in
writing the test.

Signed-off-by: Eric Blake <eblake@redhat.com>
Suggested-by: Nir Soffer <nsoffer@redhat.com>
Suggested-by: Vladimir Sementsov-Ogievskiy <v.sementsov-og@mail.ru>
Message-Id: <20220512004924.417153-3-eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
2022-05-12 13:10:52 +02:00

284 lines
7.7 KiB
C

/*
* Serving QEMU block devices via NBD
*
* Copyright (c) 2012 Red Hat, Inc.
*
* Author: Paolo Bonzini <pbonzini@redhat.com>
*
* 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 "sysemu/blockdev.h"
#include "sysemu/block-backend.h"
#include "hw/block/block.h"
#include "qapi/error.h"
#include "qapi/clone-visitor.h"
#include "qapi/qapi-visit-block-export.h"
#include "qapi/qapi-commands-block-export.h"
#include "block/nbd.h"
#include "io/channel-socket.h"
#include "io/net-listener.h"
typedef struct NBDServerData {
QIONetListener *listener;
QCryptoTLSCreds *tlscreds;
char *tlsauthz;
uint32_t max_connections;
uint32_t connections;
} NBDServerData;
static NBDServerData *nbd_server;
static int qemu_nbd_connections = -1; /* Non-negative if this is qemu-nbd */
static void nbd_update_server_watch(NBDServerData *s);
void nbd_server_is_qemu_nbd(int max_connections)
{
qemu_nbd_connections = max_connections;
}
bool nbd_server_is_running(void)
{
return nbd_server || qemu_nbd_connections >= 0;
}
int nbd_server_max_connections(void)
{
return nbd_server ? nbd_server->max_connections : qemu_nbd_connections;
}
static void nbd_blockdev_client_closed(NBDClient *client, bool ignored)
{
nbd_client_put(client);
assert(nbd_server->connections > 0);
nbd_server->connections--;
nbd_update_server_watch(nbd_server);
}
static void nbd_accept(QIONetListener *listener, QIOChannelSocket *cioc,
gpointer opaque)
{
nbd_server->connections++;
nbd_update_server_watch(nbd_server);
qio_channel_set_name(QIO_CHANNEL(cioc), "nbd-server");
nbd_client_new(cioc, nbd_server->tlscreds, nbd_server->tlsauthz,
nbd_blockdev_client_closed);
}
static void nbd_update_server_watch(NBDServerData *s)
{
if (!s->max_connections || s->connections < s->max_connections) {
qio_net_listener_set_client_func(s->listener, nbd_accept, NULL, NULL);
} else {
qio_net_listener_set_client_func(s->listener, NULL, NULL, NULL);
}
}
static void nbd_server_free(NBDServerData *server)
{
if (!server) {
return;
}
qio_net_listener_disconnect(server->listener);
object_unref(OBJECT(server->listener));
if (server->tlscreds) {
object_unref(OBJECT(server->tlscreds));
}
g_free(server->tlsauthz);
g_free(server);
}
static QCryptoTLSCreds *nbd_get_tls_creds(const char *id, Error **errp)
{
Object *obj;
QCryptoTLSCreds *creds;
obj = object_resolve_path_component(
object_get_objects_root(), id);
if (!obj) {
error_setg(errp, "No TLS credentials with id '%s'",
id);
return NULL;
}
creds = (QCryptoTLSCreds *)
object_dynamic_cast(obj, TYPE_QCRYPTO_TLS_CREDS);
if (!creds) {
error_setg(errp, "Object with id '%s' is not TLS credentials",
id);
return NULL;
}
if (!qcrypto_tls_creds_check_endpoint(creds,
QCRYPTO_TLS_CREDS_ENDPOINT_SERVER,
errp)) {
return NULL;
}
object_ref(obj);
return creds;
}
void nbd_server_start(SocketAddress *addr, const char *tls_creds,
const char *tls_authz, uint32_t max_connections,
Error **errp)
{
if (nbd_server) {
error_setg(errp, "NBD server already running");
return;
}
nbd_server = g_new0(NBDServerData, 1);
nbd_server->max_connections = max_connections;
nbd_server->listener = qio_net_listener_new();
qio_net_listener_set_name(nbd_server->listener,
"nbd-listener");
/*
* Because this server is persistent, a backlog of SOMAXCONN is
* better than trying to size it to max_connections.
*/
if (qio_net_listener_open_sync(nbd_server->listener, addr, SOMAXCONN,
errp) < 0) {
goto error;
}
if (tls_creds) {
nbd_server->tlscreds = nbd_get_tls_creds(tls_creds, errp);
if (!nbd_server->tlscreds) {
goto error;
}
}
nbd_server->tlsauthz = g_strdup(tls_authz);
nbd_update_server_watch(nbd_server);
return;
error:
nbd_server_free(nbd_server);
nbd_server = NULL;
}
void nbd_server_start_options(NbdServerOptions *arg, Error **errp)
{
nbd_server_start(arg->addr, arg->tls_creds, arg->tls_authz,
arg->max_connections, errp);
}
void qmp_nbd_server_start(SocketAddressLegacy *addr,
bool has_tls_creds, const char *tls_creds,
bool has_tls_authz, const char *tls_authz,
bool has_max_connections, uint32_t max_connections,
Error **errp)
{
SocketAddress *addr_flat = socket_address_flatten(addr);
nbd_server_start(addr_flat, tls_creds, tls_authz, max_connections, errp);
qapi_free_SocketAddress(addr_flat);
}
void qmp_nbd_server_add(NbdServerAddOptions *arg, Error **errp)
{
BlockExport *export;
BlockDriverState *bs;
BlockBackend *on_eject_blk;
BlockExportOptions *export_opts;
bs = bdrv_lookup_bs(arg->device, arg->device, errp);
if (!bs) {
return;
}
/*
* block-export-add would default to the node-name, but we may have to use
* the device name as a default here for compatibility.
*/
if (!arg->has_name) {
arg->has_name = true;
arg->name = g_strdup(arg->device);
}
export_opts = g_new(BlockExportOptions, 1);
*export_opts = (BlockExportOptions) {
.type = BLOCK_EXPORT_TYPE_NBD,
.id = g_strdup(arg->name),
.node_name = g_strdup(bdrv_get_node_name(bs)),
.has_writable = arg->has_writable,
.writable = arg->writable,
};
QAPI_CLONE_MEMBERS(BlockExportOptionsNbdBase, &export_opts->u.nbd,
qapi_NbdServerAddOptions_base(arg));
if (arg->has_bitmap) {
BlockDirtyBitmapOrStr *el = g_new(BlockDirtyBitmapOrStr, 1);
*el = (BlockDirtyBitmapOrStr) {
.type = QTYPE_QSTRING,
.u.local = g_strdup(arg->bitmap),
};
export_opts->u.nbd.has_bitmaps = true;
QAPI_LIST_PREPEND(export_opts->u.nbd.bitmaps, el);
}
/*
* nbd-server-add doesn't complain when a read-only device should be
* exported as writable, but simply downgrades it. This is an error with
* block-export-add.
*/
if (bdrv_is_read_only(bs)) {
export_opts->has_writable = true;
export_opts->writable = false;
}
export = blk_exp_add(export_opts, errp);
if (!export) {
goto fail;
}
/*
* nbd-server-add removes the export when the named BlockBackend used for
* @device goes away.
*/
on_eject_blk = blk_by_name(arg->device);
if (on_eject_blk) {
nbd_export_set_on_eject_blk(export, on_eject_blk);
}
fail:
qapi_free_BlockExportOptions(export_opts);
}
void qmp_nbd_server_remove(const char *name,
bool has_mode, BlockExportRemoveMode mode,
Error **errp)
{
BlockExport *exp;
exp = blk_exp_find(name);
if (exp && exp->drv->type != BLOCK_EXPORT_TYPE_NBD) {
error_setg(errp, "Block export '%s' is not an NBD export", name);
return;
}
qmp_block_export_del(name, has_mode, mode, errp);
}
void qmp_nbd_server_stop(Error **errp)
{
if (!nbd_server) {
error_setg(errp, "NBD server not running");
return;
}
blk_exp_close_all_type(BLOCK_EXPORT_TYPE_NBD);
nbd_server_free(nbd_server);
nbd_server = NULL;
}