Re: [Qemu-devel] [PATCH v4 08/13] vfio: Add save state functions to SaveVMHandlers

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Kirti Wankhede (kwankhede@nvidia.com) wrote:
> Added .save_live_pending, .save_live_iterate and .save_live_complete_precopy
> functions. These functions handles pre-copy and stop-and-copy phase.
> 
> In _SAVING|_RUNNING device state or pre-copy phase:
> - read pending_bytes
> - read data_offset - indicates kernel driver to write data to staging
>   buffer which is mmapped.
> - read data_size - amount of data in bytes written by vendor driver in migration
>   region.
> - if data section is trapped, pread() number of bytes in data_size, from
>   data_offset.
> - if data section is mmaped, read mmaped buffer of size data_size.
> - Write data packet to file stream as below:
> {VFIO_MIG_FLAG_DEV_DATA_STATE, data_size, actual data,
> VFIO_MIG_FLAG_END_OF_STATE }
> 
> In _SAVING device state or stop-and-copy phase
> a. read config space of device and save to migration file stream. This
>    doesn't need to be from vendor driver. Any other special config state
>    from driver can be saved as data in following iteration.
> b. read pending_bytes - indicates kernel driver to write data to staging
>    buffer which is mmapped.
> c. read data_size - amount of data in bytes written by vendor driver in
>    migration region.
> d. if data section is trapped, pread() from data_offset of size data_size.
> e. if data section is mmaped, read mmaped buffer of size data_size.
> f. Write data packet as below:
>    {VFIO_MIG_FLAG_DEV_DATA_STATE, data_size, actual data}
> g. iterate through steps b to f until (pending_bytes > 0)
> h. Write {VFIO_MIG_FLAG_END_OF_STATE}
> 
> .save_live_iterate runs outside the iothread lock in the migration case, which
> could race with asynchronous call to get dirty page list causing data corruption
> in mapped migration region. Mutex added here to serial migration buffer read
> operation.
> 
> Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
> Reviewed-by: Neo Jia <cjia@nvidia.com>
> ---
>  hw/vfio/migration.c | 212 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 212 insertions(+)
> 
> diff --git a/hw/vfio/migration.c b/hw/vfio/migration.c
> index fe0887c27664..0a2f30872316 100644
> --- a/hw/vfio/migration.c
> +++ b/hw/vfio/migration.c
> @@ -107,6 +107,111 @@ static int vfio_migration_set_state(VFIODevice *vbasedev, uint32_t state)
>      return 0;
>  }
>  
> +static int vfio_save_buffer(QEMUFile *f, VFIODevice *vbasedev)
> +{
> +    VFIOMigration *migration = vbasedev->migration;
> +    VFIORegion *region = &migration->region.buffer;
> +    uint64_t data_offset = 0, data_size = 0;
> +    int ret;
> +
> +    ret = pread(vbasedev->fd, &data_offset, sizeof(data_offset),
> +                region->fd_offset + offsetof(struct vfio_device_migration_info,
> +                                             data_offset));
> +    if (ret != sizeof(data_offset)) {
> +        error_report("Failed to get migration buffer data offset %d",
> +                     ret);
> +        return -EINVAL;
> +    }

It feels like you need a helper function, something so that you can do
something like:

       if (!vfio_dev_read(vbasedev, &data_offset, sizeof(data_offset),
                          region->fd_offset + offsetof(struct vfio_device_migration_info,
                                                data_offset),
                          "data offset")) {
           return -EINVAL;
       }

> +    ret = pread(vbasedev->fd, &data_size, sizeof(data_size),
> +                region->fd_offset + offsetof(struct vfio_device_migration_info,
> +                                             data_size));
> +    if (ret != sizeof(data_size)) {
> +        error_report("Failed to get migration buffer data size %d",
> +                     ret);
> +        return -EINVAL;
> +    }
> +
> +    if (data_size > 0) {
> +        void *buf = NULL;
> +        bool buffer_mmaped = false;
> +
> +        if (region->mmaps) {
> +            int i;
> +
> +            for (i = 0; i < region->nr_mmaps; i++) {
> +                if ((data_offset >= region->mmaps[i].offset) &&
> +                    (data_offset < region->mmaps[i].offset +
> +                                   region->mmaps[i].size)) {
> +                    buf = region->mmaps[i].mmap + (data_offset -
> +                                                   region->mmaps[i].offset);
> +                    buffer_mmaped = true;
> +                    break;
> +                }
> +            }
> +        }
> +
> +        if (!buffer_mmaped) {
> +            buf = g_malloc0(data_size);
> +            ret = pread(vbasedev->fd, buf, data_size,
> +                        region->fd_offset + data_offset);
> +            if (ret != data_size) {
> +                error_report("Failed to get migration data %d", ret);
> +                g_free(buf);
> +                return -EINVAL;
> +            }
> +        }
> +
> +        qemu_put_be64(f, data_size);
> +        qemu_put_buffer(f, buf, data_size);
> +
> +        if (!buffer_mmaped) {
> +            g_free(buf);
> +        }
> +        migration->pending_bytes -= data_size;
> +    } else {
> +        qemu_put_be64(f, data_size);
> +    }
> +
> +    ret = qemu_file_get_error(f);

You're ignoring that return value;  it's not that
important to check for errors on the saving side - although
you should if you're looping on data to fail quickly; it's more
of an issue on the load side.

> +    return data_size;
> +}
> +
> +static int vfio_update_pending(VFIODevice *vbasedev)
> +{
> +    VFIOMigration *migration = vbasedev->migration;
> +    VFIORegion *region = &migration->region.buffer;
> +    uint64_t pending_bytes = 0;
> +    int ret;
> +
> +    ret = pread(vbasedev->fd, &pending_bytes, sizeof(pending_bytes),
> +                region->fd_offset + offsetof(struct vfio_device_migration_info,
> +                                             pending_bytes));
> +    if ((ret < 0) || (ret != sizeof(pending_bytes))) {
> +        error_report("Failed to get pending bytes %d", ret);
> +        migration->pending_bytes = 0;
> +        return (ret < 0) ? ret : -EINVAL;
> +    }
> +
> +    migration->pending_bytes = pending_bytes;
> +    return 0;
> +}
> +
> +static int vfio_save_device_config_state(QEMUFile *f, void *opaque)
> +{
> +    VFIODevice *vbasedev = opaque;
> +
> +    qemu_put_be64(f, VFIO_MIG_FLAG_DEV_CONFIG_STATE);
> +
> +    if (vbasedev->type == VFIO_DEVICE_TYPE_PCI) {
> +        vfio_pci_save_config(vbasedev, f);
> +    }
> +    qemu_put_be64(f, VFIO_MIG_FLAG_END_OF_STATE);
> +
> +    return qemu_file_get_error(f);
> +}
> +
>  /* ---------------------------------------------------------------------- */
>  
>  static int vfio_save_setup(QEMUFile *f, void *opaque)
> @@ -163,9 +268,116 @@ static void vfio_save_cleanup(void *opaque)
>      }
>  }
>  
> +static void vfio_save_pending(QEMUFile *f, void *opaque,
> +                              uint64_t threshold_size,
> +                              uint64_t *res_precopy_only,
> +                              uint64_t *res_compatible,
> +                              uint64_t *res_postcopy_only)
> +{
> +    VFIODevice *vbasedev = opaque;
> +    VFIOMigration *migration = vbasedev->migration;
> +    int ret;
> +
> +    ret = vfio_update_pending(vbasedev);
> +    if (ret) {
> +        return;
> +    }
> +
> +    if (vbasedev->device_state & VFIO_DEVICE_STATE_RUNNING) {
> +        *res_precopy_only += migration->pending_bytes;
> +    } else {
> +        *res_postcopy_only += migration->pending_bytes;
> +    }
> +    *res_compatible += 0;
> +}
> +
> +static int vfio_save_iterate(QEMUFile *f, void *opaque)
> +{
> +    VFIODevice *vbasedev = opaque;
> +    VFIOMigration *migration = vbasedev->migration;
> +    int ret;
> +
> +    qemu_put_be64(f, VFIO_MIG_FLAG_DEV_DATA_STATE);
> +
> +    qemu_mutex_lock(&migration->lock);
> +    ret = vfio_save_buffer(f, vbasedev);
> +    qemu_mutex_unlock(&migration->lock);
> +
> +    if (ret < 0) {
> +        error_report("vfio_save_buffer failed %s",
> +                     strerror(errno));
> +        return ret;
> +    }
> +
> +    qemu_put_be64(f, VFIO_MIG_FLAG_END_OF_STATE);
> +
> +    ret = qemu_file_get_error(f);
> +    if (ret) {
> +        return ret;
> +    }
> +
> +    return ret;
> +}
> +
> +static int vfio_save_complete_precopy(QEMUFile *f, void *opaque)
> +{
> +    VFIODevice *vbasedev = opaque;
> +    VFIOMigration *migration = vbasedev->migration;
> +    int ret;
> +
> +    ret = vfio_migration_set_state(vbasedev, VFIO_DEVICE_STATE_SAVING);
> +    if (ret) {
> +        error_report("Failed to set state STOP and SAVING");
> +        return ret;
> +    }
> +
> +    ret = vfio_save_device_config_state(f, opaque);
> +    if (ret) {
> +        return ret;
> +    }
> +
> +    ret = vfio_update_pending(vbasedev);
> +    if (ret) {
> +        return ret;
> +    }
> +
> +    while (migration->pending_bytes > 0) {
> +        qemu_put_be64(f, VFIO_MIG_FLAG_DEV_DATA_STATE);
> +        ret = vfio_save_buffer(f, vbasedev);
> +        if (ret < 0) {
> +            error_report("Failed to save buffer");
> +            return ret;
> +        } else if (ret == 0) {
> +            break;
> +        }
> +
> +        ret = vfio_update_pending(vbasedev);
> +        if (ret) {
> +            return ret;
> +        }
> +    }
> +
> +    qemu_put_be64(f, VFIO_MIG_FLAG_END_OF_STATE);
> +
> +    ret = qemu_file_get_error(f);
> +    if (ret) {
> +        return ret;
> +    }
> +
> +    ret = vfio_migration_set_state(vbasedev, VFIO_DEVICE_STATE_STOPPED);
> +    if (ret) {
> +        error_report("Failed to set state STOPPED");
> +        return ret;
> +    }
> +    return ret;
> +}
> +
>  static SaveVMHandlers savevm_vfio_handlers = {
>      .save_setup = vfio_save_setup,
>      .save_cleanup = vfio_save_cleanup,
> +    .save_live_pending = vfio_save_pending,
> +    .save_live_iterate = vfio_save_iterate,
> +    .save_live_complete_precopy = vfio_save_complete_precopy,
>  };
>  
>  /* ---------------------------------------------------------------------- */
> -- 
> 2.7.0
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK