Re: [Qemu-devel] [PATCH v4 29/38] blockdev: Add blockdev-insert-medium

[Wen Congyang <wency@cn.fujitsu.com>]

On 09/09/2015 08:59 PM, Max Reitz wrote:
> On 09.09.2015 12:01, Wen Congyang wrote:
>> On 09/09/2015 05:20 AM, Max Reitz wrote:
>>> On 08.09.2015 11:13, Wen Congyang wrote:
>>>> On 07/21/2015 01:45 AM, Max Reitz wrote:
>>>>> And a helper function for that, which directly takes a pointer to the
>>>>> BDS to be inserted instead of its node-name (which will be used for
>>>>> implementing 'change' using blockdev-insert-medium).
>>>>>
>>>>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>>>>> ---
>>>>>  blockdev.c           | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
>>>>>  qapi/block-core.json | 17 +++++++++++++++++
>>>>>  qmp-commands.hx      | 37 +++++++++++++++++++++++++++++++++++++
>>>>>  3 files changed, 102 insertions(+)
>>>>>
>>>>> diff --git a/blockdev.c b/blockdev.c
>>>>> index 481760a..a80d0e2 100644
>>>>> --- a/blockdev.c
>>>>> +++ b/blockdev.c
>>>>> @@ -2164,6 +2164,54 @@ void qmp_blockdev_remove_medium(const char *device, Error **errp)
>>>>>      }
>>>>>  }
>>>>>  
>>>>> +static void qmp_blockdev_insert_anon_medium(const char *device,
>>>>> +                                            BlockDriverState *bs, Error **errp)
>>>>> +{
>>>>> +    BlockBackend *blk;
>>>>> +    bool has_device;
>>>>> +
>>>>> +    blk = blk_by_name(device);
>>>>> +    if (!blk) {
>>>>> +        error_set(errp, ERROR_CLASS_DEVICE_NOT_FOUND,
>>>>> +                  "Device '%s' not found", device);
>>>>> +        return;
>>>>> +    }
>>>>> +
>>>>> +    /* For BBs without a device, we can exchange the BDS tree at will */
>>>>> +    has_device = blk_get_attached_dev(blk);
>>>>> +
>>>>> +    if (has_device && !blk_dev_has_removable_media(blk)) {
>>>>> +        error_setg(errp, "Device '%s' is not removable", device);
>>>>> +        return;
>>>>> +    }
>>>>> +
>>>>> +    if (has_device && !blk_dev_is_tray_open(blk)) {
>>>>> +        error_setg(errp, "Tray of device '%s' is not open", device);
>>>>> +        return;
>>>>> +    }
>>>>> +
>>>>> +    if (blk_bs(blk)) {
>>>>> +        error_setg(errp, "There already is a medium in device '%s'", device);
>>>>> +        return;
>>>>> +    }
>>>>> +
>>>>> +    blk_insert_bs(blk, bs);
>>>>> +}
>>>>> +
>>>>> +void qmp_blockdev_insert_medium(const char *device, const char *node_name,
>>>>> +                                Error **errp)
>>>>> +{
>>>>> +    BlockDriverState *bs;
>>>>> +
>>>>> +    bs = bdrv_find_node(node_name);
>>>>> +    if (!bs) {
>>>>> +        error_setg(errp, "Node '%s' not found", node_name);
>>>>> +        return;
>>>>> +    }
>>>>
>>>> Hmm, it is OK if the bs is not top BDS?
>>>
>>> I think so, yes. Generally, there's probably no reason to do that, but I
>>> don't know why we should not allow that case. For instance, you might
>>> want to make a backing file available read-only somewhere.
>>>
>>> It should be impossible to make it available writable, and it should not
>>> be allowed to start a block-commit operation while the backing file can
>>> be accessed by the guest, but this should be achieved using op blockers.
>>>
>>> What we need for this to work are fine-grained op blockers, I think. But
>>> working around that for now by only allowing to insert top BDS won't
>>> work, since you can still start block jobs which target top BDS, too
>>> (e.g. blockdev-backup can write to a BDS/BB that is visible to the guest).
>>>
>>> All in all, I think it's fine to insert non-top BDS, but we should
>>> definitely worry about which exact BDS one can insert once we have
>>> fine-grained op blockers.
>>
>> A BDS can be written by its parent, its block backend, a block job..
>> So I think we should have some way to avoid more than two sources writing
>> to it, otherwise the data may be corrupted.
> 
> Yes, and that would be op blockers.
> 
> As I said, using blockdev-backup you can write to a BB that can be
> written to by the guest as well. I think this is a bug, but it is a bug
> that needs to be fixed by having better op blockers in place, which Jeff
> Cody is working on.
> 
> Regarding this series, I don't consider this to be too big of an issue.
> Yes, if you are working with floppy disks, you can have the case of a
> block job and the guest writing to the BDS at the same time. But I can't
> really imagine who would use floppy disks and block jobs at the same
> time (people who still use floppy disks for their VMs don't strike me as
> the kind of people who use the management features of qemu, especially
> not for those floppy disks).
> 
> Other than that, this function (blockdev-insert-medium) can only be used
> for optical ROM devices (I don't think we have CD/DVD-RW support, do
> we?), so it's much less of an issue there.
> 
> So all in all I don't consider this too big of an issue here. If others
> think different, then I would delay this part of the series (which
> overhauls the "change" command) until we have fine-grained op blockers.

In most cases, the user uses this command to change CD/DVD media, so it is OK.
But IIRC scsi disk can also be changed. So we can mark this command as experimental
(the command name can be x-blockdev-insert-medium).

Thanks
Wen Congyang

> 
> Max
>