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

[Wenchao Xia]

于 2013-5-28 15:46, Stefan Hajnoczi 写道:
> 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 <address@hidden>
> > >
> > > 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 <address@hidden>
> > > Reviewed-by: Eric Blake <address@hidden>
> > > Reviewed-by: Wenchao Xia <address@hidden>
> > > Signed-off-by: Wenchao Xia <address@hidden>
> > > ---
> > >   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
  Reading the code, original logic actually forbidded saving vmstate
into a removable device, now it is possible since find_vmstate_bs()
doesn't check it. How about forbid again in find_vmstate_bs()?

--
Best Regards

Wenchao Xia