Re: [Qemu-devel] [PATCH v3] virtio-blk: set correct config size for the host driver

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

On Tue, Feb 12, 2019 at 11:19:49PM +0800, Changpeng Liu wrote:
> Commit caa1ee43 "vhost-user-blk: add discard/write zeroes features
> support" added fields to struct virtio_blk_config. This changes
> the size of the config space and breaks migration from QEMU 3.1
> and older:
> 
> qemu-system-ppc64: get_pci_config_device: Bad config data: i=0x10 read: 41 device: 1 cmask: ff wmask: 80 w1cmask:0
> qemu-system-ppc64: Failed to load PCIDevice:config
> qemu-system-ppc64: Failed to load virtio-blk:virtio
> qemu-system-ppc64: error while loading state for instance 0x0 of device 'pci@800000020000000:01.0/virtio-blk'
> qemu-system-ppc64: load of migration failed: Invalid argument
> 
> Since virtio-blk doesn't support the "discard" and "write zeroes"
> features, it shouldn't even expose the associated fields in the
> config space actually. Just include all fields up to num_queues to
> match QEMU 3.1 and older.
> 
> Signed-off-by: Changpeng Liu <changpeng.liu@intel.com>

OK almost.

> ---
>  hw/block/virtio-blk.c | 14 ++++++++++----
>  1 file changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
> index 9a87b3b..0ff5315 100644
> --- a/hw/block/virtio-blk.c
> +++ b/hw/block/virtio-blk.c
> @@ -28,6 +28,10 @@
>  #include "hw/virtio/virtio-bus.h"
>  #include "hw/virtio/virtio-access.h"
>  
> +/* We don't support discard yet, hide associated config fields. */
> +#define VIRTIO_BLK_CFG_SIZE offsetof(struct virtio_blk_config, \
> +                                     max_discard_sectors)
> +
>  static void virtio_blk_init_request(VirtIOBlock *s, VirtQueue *vq,
>                                      VirtIOBlockReq *req)
>  {
> @@ -761,7 +765,9 @@ 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, VIRTIO_BLK_CFG_SIZE);
> +    QEMU_BUILD_BUG_ON(VIRTIO_BLK_CFG_SIZE > sizeof(struct virtio_blk_config));
> +

Oh probably sizeof blkcfg here, right?
Also we don't need the empty line here.

>  }
>  
>  static void virtio_blk_set_config(VirtIODevice *vdev, const uint8_t *config)
> @@ -769,7 +775,8 @@ 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, VIRTIO_BLK_CFG_SIZE);
> +    QEMU_BUILD_BUG_ON(VIRTIO_BLK_CFG_SIZE > sizeof(blkcfg));
>  
>      aio_context_acquire(blk_get_aio_context(s->blk));
>      blk_set_enable_write_cache(s->blk, blkcfg.wce != 0);
> @@ -952,8 +959,7 @@ 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_init(vdev, "virtio-blk", VIRTIO_ID_BLOCK, VIRTIO_BLK_CFG_SIZE);
>  
>      s->blk = conf->conf.blk;
>      s->rq = NULL;
> -- 
> 1.9.3