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 22:03, 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;
> +        }
> +    }
> +
> +    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 = {