vfio/migration: Multifd device state transfer support - received buffers queuing

The multifd received data needs to be reassembled since device state
packets sent via different multifd channels can arrive out-of-order.

Therefore, each VFIO device state packet carries a header indicating its
position in the stream.
The raw device state data is saved into a VFIOStateBuffer for later
in-order loading into the device.

The last such VFIO device state packet should have
VFIO_DEVICE_STATE_CONFIG_STATE flag set and carry the device config state.

Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/e3bff515a8d61c582b94b409eb12a45b1a143a69.1741124640.git.maciej.szmigiero@oracle.com
[ clg: - Reordered savevm_vfio_handlers
       - Added load_state_buffer documentation ]
Signed-off-by: Cédric Le Goater <clg@redhat.com>

docs/devel/migration/vfio.rst

@@ -76,6 +76,10 @@ VFIO implements the device hooks for the iterative approach as follows:
 * A ``load_state`` function that loads the config section and the data
   sections that are generated by the save functions above.
 
+* A ``load_state_buffer`` function that loads the device state and the device
+  config that arrived via multifd channels.
+  It's used only in the multifd mode.
+
 * ``cleanup`` functions for both save and load that perform any migration
   related cleanup.
 
@@ -194,6 +198,9 @@ Live migration resume path
                           (RESTORE_VM, _ACTIVE, _STOP)
                                       |
      For each device, .load_state() is called for that device section data
+                 transmitted via the main migration channel.
+     For data transmitted via multifd channels .load_state_buffer() is called
+                                   instead.
                         (RESTORE_VM, _ACTIVE, _RESUMING)
                                       |
   At the end, .load_cleanup() is called for each device and vCPUs are started

hw/vfio/migration-multifd.c

@@ -32,18 +32,181 @@ typedef struct VFIODeviceStatePacket {
     uint8_t data[0];
 } QEMU_PACKED VFIODeviceStatePacket;
 
+/* type safety */
+typedef struct VFIOStateBuffers {
+    GArray *array;
+} VFIOStateBuffers;
+
+typedef struct VFIOStateBuffer {
+    bool is_present;
+    char *data;
+    size_t len;
+} VFIOStateBuffer;
+
 typedef struct VFIOMultifd {
+    VFIOStateBuffers load_bufs;
+    QemuCond load_bufs_buffer_ready_cond;
+    QemuMutex load_bufs_mutex; /* Lock order: this lock -> BQL */
+    uint32_t load_buf_idx;
+    uint32_t load_buf_idx_last;
 } VFIOMultifd;
 
+static void vfio_state_buffer_clear(gpointer data)
+{
+    VFIOStateBuffer *lb = data;
+
+    if (!lb->is_present) {
+        return;
+    }
+
+    g_clear_pointer(&lb->data, g_free);
+    lb->is_present = false;
+}
+
+static void vfio_state_buffers_init(VFIOStateBuffers *bufs)
+{
+    bufs->array = g_array_new(FALSE, TRUE, sizeof(VFIOStateBuffer));
+    g_array_set_clear_func(bufs->array, vfio_state_buffer_clear);
+}
+
+static void vfio_state_buffers_destroy(VFIOStateBuffers *bufs)
+{
+    g_clear_pointer(&bufs->array, g_array_unref);
+}
+
+static void vfio_state_buffers_assert_init(VFIOStateBuffers *bufs)
+{
+    assert(bufs->array);
+}
+
+static unsigned int vfio_state_buffers_size_get(VFIOStateBuffers *bufs)
+{
+    return bufs->array->len;
+}
+
+static void vfio_state_buffers_size_set(VFIOStateBuffers *bufs,
+                                        unsigned int size)
+{
+    g_array_set_size(bufs->array, size);
+}
+
+static VFIOStateBuffer *vfio_state_buffers_at(VFIOStateBuffers *bufs,
+                                              unsigned int idx)
+{
+    return &g_array_index(bufs->array, VFIOStateBuffer, idx);
+}
+
+/* called with load_bufs_mutex locked */
+static bool vfio_load_state_buffer_insert(VFIODevice *vbasedev,
+                                          VFIODeviceStatePacket *packet,
+                                          size_t packet_total_size,
+                                          Error **errp)
+{
+    VFIOMigration *migration = vbasedev->migration;
+    VFIOMultifd *multifd = migration->multifd;
+    VFIOStateBuffer *lb;
+
+    vfio_state_buffers_assert_init(&multifd->load_bufs);
+    if (packet->idx >= vfio_state_buffers_size_get(&multifd->load_bufs)) {
+        vfio_state_buffers_size_set(&multifd->load_bufs, packet->idx + 1);
+    }
+
+    lb = vfio_state_buffers_at(&multifd->load_bufs, packet->idx);
+    if (lb->is_present) {
+        error_setg(errp, "%s: state buffer %" PRIu32 " already filled",
+                   vbasedev->name, packet->idx);
+        return false;
+    }
+
+    assert(packet->idx >= multifd->load_buf_idx);
+
+    lb->data = g_memdup2(&packet->data, packet_total_size - sizeof(*packet));
+    lb->len = packet_total_size - sizeof(*packet);
+    lb->is_present = true;
+
+    return true;
+}
+
+bool vfio_multifd_load_state_buffer(void *opaque, char *data, size_t data_size,
+                                    Error **errp)
+{
+    VFIODevice *vbasedev = opaque;
+    VFIOMigration *migration = vbasedev->migration;
+    VFIOMultifd *multifd = migration->multifd;
+    VFIODeviceStatePacket *packet = (VFIODeviceStatePacket *)data;
+
+    if (!vfio_multifd_transfer_enabled(vbasedev)) {
+        error_setg(errp,
+                   "%s: got device state packet but not doing multifd transfer",
+                   vbasedev->name);
+        return false;
+    }
+
+    assert(multifd);
+
+    if (data_size < sizeof(*packet)) {
+        error_setg(errp, "%s: packet too short at %zu (min is %zu)",
+                   vbasedev->name, data_size, sizeof(*packet));
+        return false;
+    }
+
+    if (packet->version != VFIO_DEVICE_STATE_PACKET_VER_CURRENT) {
+        error_setg(errp, "%s: packet has unknown version %" PRIu32,
+                   vbasedev->name, packet->version);
+        return false;
+    }
+
+    if (packet->idx == UINT32_MAX) {
+        error_setg(errp, "%s: packet index is invalid", vbasedev->name);
+        return false;
+    }
+
+    trace_vfio_load_state_device_buffer_incoming(vbasedev->name, packet->idx);
+
+    /*
+     * Holding BQL here would violate the lock order and can cause
+     * a deadlock once we attempt to lock load_bufs_mutex below.
+     */
+    assert(!bql_locked());
+
+    WITH_QEMU_LOCK_GUARD(&multifd->load_bufs_mutex) {
+        /* config state packet should be the last one in the stream */
+        if (packet->flags & VFIO_DEVICE_STATE_CONFIG_STATE) {
+            multifd->load_buf_idx_last = packet->idx;
+        }
+
+        if (!vfio_load_state_buffer_insert(vbasedev, packet, data_size,
+                                           errp)) {
+            return false;
+        }
+
+        qemu_cond_signal(&multifd->load_bufs_buffer_ready_cond);
+    }
+
+    return true;
+}
+
 static VFIOMultifd *vfio_multifd_new(void)
 {
     VFIOMultifd *multifd = g_new(VFIOMultifd, 1);
 
+    vfio_state_buffers_init(&multifd->load_bufs);
+
+    qemu_mutex_init(&multifd->load_bufs_mutex);
+
+    multifd->load_buf_idx = 0;
+    multifd->load_buf_idx_last = UINT32_MAX;
+    qemu_cond_init(&multifd->load_bufs_buffer_ready_cond);
+
     return multifd;
 }
 
 static void vfio_multifd_free(VFIOMultifd *multifd)
 {
+    vfio_state_buffers_destroy(&multifd->load_bufs);
+    qemu_cond_destroy(&multifd->load_bufs_buffer_ready_cond);
+    qemu_mutex_destroy(&multifd->load_bufs_mutex);
+
     g_free(multifd);
 }
 

hw/vfio/migration-multifd.h

@@ -20,4 +20,7 @@ void vfio_multifd_cleanup(VFIODevice *vbasedev);
 bool vfio_multifd_transfer_supported(void);
 bool vfio_multifd_transfer_enabled(VFIODevice *vbasedev);
 
+bool vfio_multifd_load_state_buffer(void *opaque, char *data, size_t data_size,
+                                    Error **errp);
+
 #endif

hw/vfio/migration.c

@@ -802,6 +802,10 @@ static const SaveVMHandlers savevm_vfio_handlers = {
     .load_cleanup = vfio_load_cleanup,
     .load_state = vfio_load_state,
     .switchover_ack_needed = vfio_switchover_ack_needed,
+    /*
+     * Multifd support
+     */
+    .load_state_buffer = vfio_multifd_load_state_buffer,
 };
 
 /* ---------------------------------------------------------------------- */

hw/vfio/trace-events

@@ -154,6 +154,7 @@ vfio_load_device_config_state_start(const char *name) " (%s)"
 vfio_load_device_config_state_end(const char *name) " (%s)"
 vfio_load_state(const char *name, uint64_t data) " (%s) data 0x%"PRIx64
 vfio_load_state_device_data(const char *name, uint64_t data_size, int ret) " (%s) size %"PRIu64" ret %d"
+vfio_load_state_device_buffer_incoming(const char *name, uint32_t idx) " (%s) idx %"PRIu32
 vfio_migration_realize(const char *name) " (%s)"
 vfio_migration_set_device_state(const char *name, const char *state) " (%s) state %s"
 vfio_migration_set_state(const char *name, const char *new_state, const char *recover_state) " (%s) new state %s, recover state %s"