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

[Wenchao Xia <xiawenc@linux.vnet.ibm.com>]

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 <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?
>
> 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