Re: [Qemu-devel] [PATCH V3 1/4] block: drop bs_snapshots global variable

[Kevin Wolf <kwolf@redhat.com>]

Am 25.05.2013 um 05:09 hat Wenchao Xia geschrieben:
> From: Stefan Hajnoczi <stefanha@redhat.com>
> 
> The bs_snapshots global variable points to the BlockDriverState which
> will be used to save vmstate.  This is really a savevm.c concept but was
> moved into block.c:bdrv_snapshots() when it became clear that hotplug
> could result in a dangling pointer.
> 
> While auditing the block layer's global state I came upon bs_snapshots
> and realized that a variable is not necessary here.  Simply find the
> first BlockDriverState capable of internal snapshots each time this is
> needed.
> 
> The behavior of bdrv_snapshots() is preserved across hotplug because new
> drives are always appended to the bdrv_states list.  This means that
> calling the new find_vmstate_bs() function is idempotent - it returns
> the same BlockDriverState unless it was hot-unplugged.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> Reviewed-by: Eric Blake <eblake@redhat.com>
> Reviewed-by: Wenchao Xia <xiawenc@linux.vnet.ibm.com>
> Signed-off-by: Wenchao Xia <xiawenc@linux.vnet.ibm.com>
> ---
>  block.c               |   28 ----------------------------
>  include/block/block.h |    1 -
>  savevm.c              |   19 +++++++++++++++----
>  3 files changed, 15 insertions(+), 33 deletions(-)
> 
> diff --git a/block.c b/block.c
> index 3f87489..478a3b2 100644
> --- a/block.c
> +++ b/block.c
> @@ -99,9 +99,6 @@ static QTAILQ_HEAD(, BlockDriverState) bdrv_states =
>  static QLIST_HEAD(, BlockDriver) bdrv_drivers =
>      QLIST_HEAD_INITIALIZER(bdrv_drivers);
>  
> -/* The device to use for VM snapshots */
> -static BlockDriverState *bs_snapshots;
> -
>  /* If non-zero, use only whitelisted block drivers */
>  static int use_bdrv_whitelist;
>  
> @@ -1357,9 +1354,6 @@ void bdrv_close(BlockDriverState *bs)
>      notifier_list_notify(&bs->close_notifiers, bs);
>  
>      if (bs->drv) {
> -        if (bs == bs_snapshots) {
> -            bs_snapshots = NULL;
> -        }
>          if (bs->backing_hd) {
>              bdrv_delete(bs->backing_hd);
>              bs->backing_hd = NULL;
> @@ -1591,7 +1585,6 @@ void bdrv_delete(BlockDriverState *bs)
>  
>      bdrv_close(bs);
>  
> -    assert(bs != bs_snapshots);
>      g_free(bs);
>  }
>  
> @@ -1635,9 +1628,6 @@ void bdrv_set_dev_ops(BlockDriverState *bs, const BlockDevOps *ops,
>  {
>      bs->dev_ops = ops;
>      bs->dev_opaque = opaque;
> -    if (bdrv_dev_has_removable_media(bs) && bs == bs_snapshots) {
> -        bs_snapshots = NULL;
> -    }

This hunk isn't replaced by any other code. If I understand correctly
what it's doing, it prevented you from saving the VM state to a
removable device, which would be allowed after this patch.

Is this a change we want to make? Why isn't it described in the commit
message?

Kevin