On Mon, May 27, 2013 at 05:25:16PM +0200, Kevin Wolf wrote:
> Am 25.05.2013 um 05:09 hat Wenchao Xia geschrieben:
> > From: Stefan Hajnoczi <stefa...@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 <stefa...@redhat.com>
> > Reviewed-by: Eric Blake <ebl...@redhat.com>
> > Reviewed-by: Wenchao Xia <xiaw...@linux.vnet.ibm.com>
> > Signed-off-by: Wenchao Xia <xiaw...@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?
My understanding of this change is different. Markus is on CC so maybe
he can confirm.
The point of bs_snapshots = NULL is not to prevent you from saving
snapshots. It's simply to reset the pointer to the next snapshottable
device (used by bdrv_snapshots()).
See the bdrv_close() hunk above which does the same thing, as well as
bdrv_snapshots() which iterates bdrv_states and updates bs_snapshots.
So what this hunk does is to reset the bdrv_snapshots() iterator when a
removable device is hooked up to an emulated storage controller. It's
no longer necessary since this patch drops the global state
(bs_snapshots) and users will always iterate from scratch.
The whole stateful approach was not necessary.
Stefan
