Re: [Qemu-devel] [PATCH] virtio-blk: set correct config size for the host driver

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Feb 12, 2019 at 12:25:16PM +0800, Changpeng Liu wrote:
> Commit caa1ee43 "vhost-user-blk: add discard/write zeroes features support"
> introduced extra fields to existing struct virtio_blk_config, when
> migration was executed from older QEMU version to current head, it
> will break the migration.  While here, set the correct config size
> when initializing the host driver, for now, discard/write zeroes
> are not supported by virtio-blk host driver, so set the config
> size as before, users can change config size when adding the new
> feature bits support.
> 
> Signed-off-by: Changpeng Liu <changpeng.liu@intel.com>
> ---
>  hw/block/virtio-blk.c          | 15 +++++++++++----
>  include/hw/virtio/virtio-blk.h |  1 +
>  2 files changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
> index 9a87b3b..fac5d33 100644
> --- a/hw/block/virtio-blk.c
> +++ b/hw/block/virtio-blk.c
> @@ -761,7 +761,7 @@ static void virtio_blk_update_config(VirtIODevice *vdev, uint8_t *config)
>      blkcfg.alignment_offset = 0;
>      blkcfg.wce = blk_enable_write_cache(s->blk);
>      virtio_stw_p(vdev, &blkcfg.num_queues, s->conf.num_queues);
> -    memcpy(config, &blkcfg, sizeof(struct virtio_blk_config));
> +    memcpy(config, &blkcfg, s->config_size);
>  }
>  
>  static void virtio_blk_set_config(VirtIODevice *vdev, const uint8_t *config)
> @@ -769,7 +769,7 @@ static void virtio_blk_set_config(VirtIODevice *vdev, const uint8_t *config)
>      VirtIOBlock *s = VIRTIO_BLK(vdev);
>      struct virtio_blk_config blkcfg;
>  
> -    memcpy(&blkcfg, config, sizeof(blkcfg));
> +    memcpy(&blkcfg, config, s->config_size);
>  
>      aio_context_acquire(blk_get_aio_context(s->blk));
>      blk_set_enable_write_cache(s->blk, blkcfg.wce != 0);
> @@ -847,6 +847,13 @@ static void virtio_blk_set_status(VirtIODevice *vdev, uint8_t status)
>      }
>  }
>  
> +static void virtio_blk_set_config_size(VirtIOBlock *s)
> +{
> +    /* VIRTIO_BLK_F_MQ is supported by host driver */

It can be disabled though.

It just so happens that only the addition of max_discard_seg
crosses the next power of 2 boundary.


> +    s->config_size = offsetof(struct virtio_blk_config, num_queues) +
> +                     sizeof_field(struct virtio_blk_config, num_queues);
> +}
> +
>  static void virtio_blk_save_device(VirtIODevice *vdev, QEMUFile *f)
>  {
>      VirtIOBlock *s = VIRTIO_BLK(vdev);
> @@ -952,8 +959,8 @@ static void virtio_blk_device_realize(DeviceState *dev, Error **errp)
>          return;
>      }
>  
> -    virtio_init(vdev, "virtio-blk", VIRTIO_ID_BLOCK,
> -                sizeof(struct virtio_blk_config));
> +    virtio_blk_set_config_size(s);
> +    virtio_init(vdev, "virtio-blk", VIRTIO_ID_BLOCK, s->config_size);
>  
>      s->blk = conf->conf.blk;
>      s->rq = NULL;
> diff --git a/include/hw/virtio/virtio-blk.h b/include/hw/virtio/virtio-blk.h
> index 5117431..9181a93 100644
> --- a/include/hw/virtio/virtio-blk.h
> +++ b/include/hw/virtio/virtio-blk.h
> @@ -51,6 +51,7 @@ typedef struct VirtIOBlock {
>      void *rq;
>      QEMUBH *bh;
>      VirtIOBlkConf conf;
> +    size_t config_size;
>      unsigned short sector_mask;
>      bool original_wce;
>      VMChangeStateEntry *change;

Well assuming you are looking for a minimal change,
go further and drop config_size completely,
replace with a macro.


> -- 
> 1.9.3