Re: [PATCH v4 31/33] vfio/migration: Multifd device state transfer support - send side

["Cédric Le Goater" <clg@redhat.com>]

On 1/30/25 11:08, Maciej S. Szmigiero wrote:
> From: "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com>
> 
> Implement the multifd device state transfer via additional per-device
> thread inside save_live_complete_precopy_thread handler.
> 
> Switch between doing the data transfer in the new handler and doing it
> in the old save_state handler depending on the
> x-migration-multifd-transfer device property value.
> 
> Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
> ---
>   hw/vfio/migration.c  | 159 +++++++++++++++++++++++++++++++++++++++++++
>   hw/vfio/trace-events |   2 +
>   2 files changed, 161 insertions(+)
> 
> diff --git a/hw/vfio/migration.c b/hw/vfio/migration.c
> index 31f651ffee85..37d1c0f3d32f 100644
> --- a/hw/vfio/migration.c
> +++ b/hw/vfio/migration.c
> @@ -943,6 +943,24 @@ static int vfio_save_setup(QEMUFile *f, void *opaque, Error **errp)
>       uint64_t stop_copy_size = VFIO_MIG_DEFAULT_DATA_BUFFER_SIZE;
>       int ret;
>   
> +    /*
> +     * Make a copy of this setting at the start in case it is changed
> +     * mid-migration.
> +     */
> +    if (vbasedev->migration_multifd_transfer == ON_OFF_AUTO_AUTO) {
> +        migration->multifd_transfer = vfio_multifd_transfer_supported();
> +    } else {
> +        migration->multifd_transfer =
> +            vbasedev->migration_multifd_transfer == ON_OFF_AUTO_ON;
> +    }
> +
> +    if (migration->multifd_transfer && !vfio_multifd_transfer_supported()) {
> +        error_setg(errp,
> +                   "%s: Multifd device transfer requested but unsupported in the current config",
> +                   vbasedev->name);
> +        return -EINVAL;
> +    }

Please implement a common routine vfio_multifd_is_enabled() that can be
shared with vfio_load_setup().

> +
>       qemu_put_be64(f, VFIO_MIG_FLAG_DEV_SETUP_STATE);
>   
>       vfio_query_stop_copy_size(vbasedev, &stop_copy_size);
> @@ -1114,13 +1132,32 @@ static int vfio_save_iterate(QEMUFile *f, void *opaque)
>       return !migration->precopy_init_size && !migration->precopy_dirty_size;
>   }
>   
> +static void vfio_save_multifd_emit_dummy_eos(VFIODevice *vbasedev, QEMUFile *f)

I would prefer naming it vfio_multifd_emit_dummy_eos().

> +{
> +    VFIOMigration *migration = vbasedev->migration;
> +
> +    assert(migration->multifd_transfer);
> +
> +    /*
> +     * Emit dummy NOP data on the main migration channel since the actual
> +     * device state transfer is done via multifd channels.
> +     */
> +    qemu_put_be64(f, VFIO_MIG_FLAG_END_OF_STATE);
> +}
> +
>   static int vfio_save_complete_precopy(QEMUFile *f, void *opaque)
>   {
>       VFIODevice *vbasedev = opaque;
> +    VFIOMigration *migration = vbasedev->migration;
>       ssize_t data_size;
>       int ret;
>       Error *local_err = NULL;
>   
> +    if (migration->multifd_transfer) {
> +        vfio_save_multifd_emit_dummy_eos(vbasedev, f);
> +        return 0;
> +    }
> +
>       trace_vfio_save_complete_precopy_start(vbasedev->name);
>   
>       /* We reach here with device state STOP or STOP_COPY only */
> @@ -1146,12 +1183,133 @@ static int vfio_save_complete_precopy(QEMUFile *f, void *opaque)
>       return ret;
>   }
>   
> +static int
> +vfio_save_complete_precopy_async_thread_config_state(VFIODevice *vbasedev,
> +                                                     char *idstr,
> +                                                     uint32_t instance_id,
> +                                                     uint32_t idx)

why use 'async_thread' in the name ?

vfio_save_complete_precopy_config_state() should be enough to refer
to its caller vfio_save_complete_precopy_thread(). Please add
an 'Error **' argument too.


> +{
> +    g_autoptr(QIOChannelBuffer) bioc = NULL;
> +    g_autoptr(QEMUFile) f = NULL;
> +    int ret;
> +    g_autofree VFIODeviceStatePacket *packet = NULL;
> +    size_t packet_len;
> +
> +    bioc = qio_channel_buffer_new(0);
> +    qio_channel_set_name(QIO_CHANNEL(bioc), "vfio-device-config-save");
> +
> +    f = qemu_file_new_output(QIO_CHANNEL(bioc));
> +
> +    ret = vfio_save_device_config_state(f, vbasedev, NULL);

I would prefer that we catch the error and propagate it to the caller.

> +    if (ret) {
> +        return ret;
> +    }
> +
> +    ret = qemu_fflush(f);
> +    if (ret) {
> +        return ret;
> +    }
> +
> +    packet_len = sizeof(*packet) + bioc->usage;
> +    packet = g_malloc0(packet_len);
> +    packet->idx = idx;
> +    packet->flags = VFIO_DEVICE_STATE_CONFIG_STATE;
> +    memcpy(&packet->data, bioc->data, bioc->usage);
> +
> +    if (!multifd_queue_device_state(idstr, instance_id,
> +                                    (char *)packet, packet_len)) {
> +        return -1;
> +    }
> +
> +    qatomic_add(&bytes_transferred, packet_len);
> +
> +    return 0;
> +}
> +
> +static int vfio_save_complete_precopy_thread(char *idstr,
> +                                             uint32_t instance_id,
> +                                             bool *abort_flag,
> +                                             void *opaque)

This lacks an "Error **" argument. I am not sure what was decided
in patch 19 "migration: Add save_live_complete_precopy_thread
handler".

We should do our best to collect and propagate errors and avoid
error_report() calls. With VFIO involved, the reasons why errors
can occur are increasingly numerous, as hardware is exposed and
host drivers are involved.

I understand this is a complex request for code when this code
relies on a framework using callbacks, even more with threads.

> +{
> +    VFIODevice *vbasedev = opaque;
> +    VFIOMigration *migration = vbasedev->migration;
> +    int ret;
> +    g_autofree VFIODeviceStatePacket *packet = NULL;
> +    uint32_t idx;
> +
> +    if (!migration->multifd_transfer) {
> +        /* Nothing to do, vfio_save_complete_precopy() does the transfer. */

why would vfio_save_complete_precopy_thread be called then ? Looks
like an error to me, may be not fatal but an error report would be
good to have. no ?

> +        return 0;
> +    }
> +
> +    trace_vfio_save_complete_precopy_thread_start(vbasedev->name,
> +                                                  idstr, instance_id);
> +
> +    /* We reach here with device state STOP or STOP_COPY only */
> +    ret = vfio_migration_set_state(vbasedev, VFIO_DEVICE_STATE_STOP_COPY,
> +                                   VFIO_DEVICE_STATE_STOP, NULL);

Error missing.

> +    if (ret) {
> +        goto ret_finish;
> +    }
> +
> +    packet = g_malloc0(sizeof(*packet) + migration->data_buffer_size);
> +
> +    for (idx = 0; ; idx++) {
> +        ssize_t data_size;
> +        size_t packet_size;
> +
> +        if (qatomic_read(abort_flag)) {
> +            ret = -ECANCELED;
> +            goto ret_finish;
> +        }
> +
> +        data_size = read(migration->data_fd, &packet->data,
> +                         migration->data_buffer_size);
> +        if (data_size < 0) {
> +            ret = -errno;
> +            goto ret_finish;
> +        } else if (data_size == 0) {
> +            break;
> +        }
> +
> +        packet->idx = idx;
> +        packet_size = sizeof(*packet) + data_size;
> +
> +        if (!multifd_queue_device_state(idstr, instance_id,
> +                                        (char *)packet, packet_size)) {
> +            ret = -1;
> +            goto ret_finish;
> +        }
> +
> +        qatomic_add(&bytes_transferred, packet_size);
> +    }
> +
> +    ret = vfio_save_complete_precopy_async_thread_config_state(vbasedev, idstr,
> +                                                               instance_id,
> +                                                               idx);
> +
> +ret_finish:
> +    trace_vfio_save_complete_precopy_thread_end(vbasedev->name, ret);
> +
> +    return ret;
> +}
> +
>   static void vfio_save_state(QEMUFile *f, void *opaque)
>   {
>       VFIODevice *vbasedev = opaque;
> +    VFIOMigration *migration = vbasedev->migration;
>       Error *local_err = NULL;
>       int ret;
>   
> +    if (migration->multifd_transfer) {
> +        if (vfio_load_config_after_iter(vbasedev)) {
> +            qemu_put_be64(f, VFIO_MIG_FLAG_DEV_CONFIG_LOAD_READY);


Please put the above chunck at the end of the series with the patch
adding ARM support.

> +        } else {
> +            vfio_save_multifd_emit_dummy_eos(vbasedev, f);
> +        }

Please introduce a vfio_multifd_save_state() routine and a
vfio_"normal"_save_state() routine and change vfio_save_state()
to call one or the other.


Thanks,

C.



> +        return;
> +    }
> +
>       ret = vfio_save_device_config_state(f, opaque, &local_err);
>       if (ret) {
>           error_prepend(&local_err,
> @@ -1372,6 +1530,7 @@ static const SaveVMHandlers savevm_vfio_handlers = {
>       .is_active_iterate = vfio_is_active_iterate,
>       .save_live_iterate = vfio_save_iterate,
>       .save_live_complete_precopy = vfio_save_complete_precopy,
> +    .save_live_complete_precopy_thread = vfio_save_complete_precopy_thread,
>       .save_state = vfio_save_state,
>       .load_setup = vfio_load_setup,
>       .load_cleanup = vfio_load_cleanup,
> diff --git a/hw/vfio/trace-events b/hw/vfio/trace-events
> index 418b378ebd29..039979bdd98f 100644
> --- a/hw/vfio/trace-events
> +++ b/hw/vfio/trace-events
> @@ -168,6 +168,8 @@ vfio_save_block_precopy_empty_hit(const char *name) " (%s)"
>   vfio_save_cleanup(const char *name) " (%s)"
>   vfio_save_complete_precopy(const char *name, int ret) " (%s) ret %d"
>   vfio_save_complete_precopy_start(const char *name) " (%s)"
> +vfio_save_complete_precopy_thread_start(const char *name, const char *idstr, uint32_t instance_id) " (%s) idstr %s instance %"PRIu32
> +vfio_save_complete_precopy_thread_end(const char *name, int ret) " (%s) ret %d"
>   vfio_save_device_config_state(const char *name) " (%s)"
>   vfio_save_iterate(const char *name, uint64_t precopy_init_size, uint64_t precopy_dirty_size) " (%s) precopy initial size %"PRIu64" precopy dirty size %"PRIu64
>   vfio_save_iterate_start(const char *name) " (%s)"
>