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

[Denis Plotnikov <dplotnikov@virtuozzo.com>]

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 <dplotnikov@virtuozzo.com>
>> ---
>>   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 = {