[PING]Re: [PATCH v0 2/2] block: allow to set 'drive' property on a realized block device

[Denis Plotnikov]

On 18.11.2019 13:50, Denis Plotnikov wrote:
>
>
> On 10.11.2019 22:08, Denis Plotnikov wrote:
>>
>> On 10.11.2019 22:03, Denis Plotnikov wrote:
>>> This allows to change (replace) the file on a block device and is 
>>> useful
>>> to workaround exclusive file access restrictions, e.g. to implement VM
>>> migration with a shared disk stored on some storage with the exclusive
>>> file opening model: a destination VM is started waiting for incomming
>>> migration with a fake image drive, and later, on the last migration
>>> phase, the fake image file is replaced with the real one.
>>>
>>> Signed-off-by: Denis Plotnikov <address@hidden>
>>> ---
>>>   hw/core/qdev-properties-system.c | 89 
>>> +++++++++++++++++++++++++++-----
>>>   1 file changed, 77 insertions(+), 12 deletions(-)
>>>
>>> diff --git a/hw/core/qdev-properties-system.c 
>>> b/hw/core/qdev-properties-system.c
>>> index c534590dcd..aaab1370a4 100644
>>> --- a/hw/core/qdev-properties-system.c
>>> +++ b/hw/core/qdev-properties-system.c
>>> @@ -79,8 +79,55 @@ static void set_pointer(Object *obj, Visitor *v, 
>>> Property *prop,
>>>     /* --- drive --- */
>>>   -static void do_parse_drive(DeviceState *dev, const char *str, 
>>> void **ptr,
>>> -                           const char *propname, bool iothread, 
>>> Error **errp)
>>> +static void do_parse_drive_realized(DeviceState *dev, const char *str,
>>> +                                    void **ptr, const char *propname,
>>> +                                    bool iothread, Error **errp)
>>> +{
>>> +    BlockBackend *blk = *ptr;
>>> +    BlockDriverState *bs = bdrv_lookup_bs(NULL, str, NULL);
>>> +    int ret;
>>> +    bool blk_created = false;
>>> +
>>> +    if (!bs) {
>>> +        error_setg(errp, "Can't find blockdev '%s'", str);
>>> +        return;
>>> +    }
>>> +
>>> +    if (!blk) {
>>> +        AioContext *ctx = iothread ? bdrv_get_aio_context(bs) :
>>> +                                     qemu_get_aio_context();
>>> +        blk = blk_new(ctx, BLK_PERM_ALL, BLK_PERM_ALL);
>>> +        blk_created = true;
>>
>> Actually, I have concerns about situation where blk=null.
>>
>> Is there any case when scsi-hd (or others) doesn't have a blk 
>> assigned and it's legal?
>>
>>> +    } else {
>>> +        if (blk_bs(blk)) {
>>> +            blk_remove_bs(blk);
>>> +        }
>>> +    }
>>> +
>>> +    ret = blk_insert_bs(blk, bs, errp);
>>> +
>>> +    if (!ret && blk_created) {
>>> +        if (blk_attach_dev(blk, dev) < 0) {
>>> +            /*
>>> +             * Shouldn't be any errors here since we just created
>>> +             * the new blk because the device doesn't have any.
>>> +             * Leave the message here in case blk_attach_dev is 
>>> changed
>>> +             */
>>> +             error_setg(errp, "Can't attach drive '%s' to device 
>>> '%s'",
>>> +                        str, object_get_typename(OBJECT(dev)));
>>> +        } else {
>>> +            *ptr = blk;
>>> +        }
>>> +    }
> Another problem here, is that the "size" of the device dev may not 
> match after setting a drive.
> So, we should update it after the drive setting.
> It was found, that it could be done by calling 
> BlockDevOps.bdrv_parent_cb_resize.
>
> But I have some concerns about doing it so. In the case of virtio scsi 
> disk we have the following callstack
>
>     bdrv_parent_cb_resize calls() ->
>         scsi_device_report_change(dev, SENSE_CODE(CAPACITY_CHANGED)) ->
>             virtio_scsi_change ->
>                 virtio_scsi_push_event(s, dev, 
> VIRTIO_SCSI_T_PARAM_CHANGE,
>                                                     sense.asc | 
> (sense.ascq << 8));
>
>
> virtio_scsi_change  pushes the event to the guest to make the guest 
> ask for size refreshing.
> If I'm not mistaken, here we can get a race condition when some 
> another request is processed with an unchanged
> size and then the size changing request is processed.
>
> I didn't find a better way to update device size so any comments are 
> welcome.
>
> Thanks!
>
> Denis
>>> +
>>> +    if (blk_created) {
>>> +        blk_unref(blk);
>>> +    }
>>> +}
>>> +
>>> +static void do_parse_drive_unrealized(DeviceState *dev, const char 
>>> *str,
>>> +                                      void **ptr, const char 
>>> *propname,
>>> +                                      bool iothread, Error **errp)
>>>   {
>>>       BlockBackend *blk;
>>>       bool blk_created = false;
>>> @@ -137,18 +184,34 @@ fail:
>>>       }
>>>   }
>>>   -static void parse_drive(DeviceState *dev, const char *str, void 
>>> **ptr,
>>> -                        const char *propname, Error **errp)
>>> -{
>>> -    do_parse_drive(dev, str, ptr, propname, false, errp);
>>> -}
>>> -
>>> -static void parse_drive_iothread(DeviceState *dev, const char *str, 
>>> void **ptr,
>>> +static void parse_drive_realized(DeviceState *dev, const char *str, 
>>> void **ptr,
>>>                                    const char *propname, Error **errp)
>>>   {
>>> -    do_parse_drive(dev, str, ptr, propname, true, errp);
>>> +    do_parse_drive_realized(dev, str, ptr, propname, false, errp);
>>>   }
>>>   +static void parse_drive_realized_iothread(DeviceState *dev, const 
>>> char *str,
>>> +                                          void **ptr, const char 
>>> *propname,
>>> +                                          Error **errp)
>>> +{
>>> +    do_parse_drive_realized(dev, str, ptr, propname, true, errp);
>>> +}
>>> +
>>> +static void parse_drive_unrealized(DeviceState *dev, const char *str,
>>> +                                   void **ptr, const char *propname,
>>> +                                   Error **errp)
>>> +{
>>> +    do_parse_drive_unrealized(dev, str, ptr, propname, false, errp);
>>> +}
>>> +
>>> +static void parse_drive_unrealized_iothread(DeviceState *dev, const 
>>> char *str,
>>> +                                            void **ptr, const char 
>>> *propname,
>>> +                                            Error **errp)
>>> +{
>>> +    do_parse_drive_unrealized(dev, str, ptr, propname, true, errp);
>>> +}
>>> +
>>> +
>>>   static void release_drive(Object *obj, const char *name, void 
>>> *opaque)
>>>   {
>>>       DeviceState *dev = DEVICE(obj);
>>> @@ -188,13 +251,15 @@ static void get_drive(Object *obj, Visitor *v, 
>>> const char *name, void *opaque,
>>>   static void set_drive(Object *obj, Visitor *v, const char *name, 
>>> void *opaque,
>>>                         Error **errp)
>>>   {
>>> -    set_pointer(obj, v, opaque, NULL, parse_drive, name, errp);
>>> +    set_pointer(obj, v, opaque, parse_drive_realized, 
>>> parse_drive_unrealized,
>>> +                name, errp);
>>>   }
>>>     static void set_drive_iothread(Object *obj, Visitor *v, const 
>>> char *name,
>>>                                  void *opaque, Error **errp)
>>>   {
>>> -    set_pointer(obj, v, opaque, NULL, parse_drive_iothread, name, 
>>> errp);
>>> +    set_pointer(obj, v, opaque, parse_drive_realized_iothread,
>>> +                parse_drive_unrealized_iothread, name, errp);
>>>   }
>>>     const PropertyInfo qdev_prop_drive = {
>