Re: [Qemu-devel] [Qemu-block] [RFC] nvme: how to support multiple namespaces

[Klaus Birkelund <klaus@birkelund.eu>]

On Wed, Jun 26, 2019 at 12:14:15PM +0200, Paolo Bonzini wrote:
> On 26/06/19 06:46, Markus Armbruster wrote:
> >> I'm not sure how to wire it together without the bus abstraction? So
> >> I'll stick with the bus for now. It *is* extremely convenient!
> > 
> > As far as I can tell offhand, a common use of bus-less connections
> > between devices is wiring together composite devices.  Example:
> > 
> >     static void designware_pcie_host_init(Object *obj)
> >     {
> >         DesignwarePCIEHost *s = DESIGNWARE_PCIE_HOST(obj);
> >         DesignwarePCIERoot *root = &s->root;
> > 
> >         object_initialize_child(obj, "root",  root, sizeof(*root),
> >                                 TYPE_DESIGNWARE_PCIE_ROOT, &error_abort, NULL);
> >         qdev_prop_set_int32(DEVICE(root), "addr", PCI_DEVFN(0, 0));
> >         qdev_prop_set_bit(DEVICE(root), "multifunction", false);
> >     }
> > 
> > This creates a TYPE_DESIGNWARE_PCIE_ROOT device "within" the
> > TYPE_DESIGNWARE_PCIE_HOST device.
> > 
> > Bus-less connections between separate devices (i.e. neither device is a
> > part of the other) are also possible.  But I'm failing at grep right
> > now.  Here's an example for connecting a device to a machine:
> > 
> >     static void mch_realize(PCIDevice *d, Error **errp)
> >     {
> >         int i;
> >         MCHPCIState *mch = MCH_PCI_DEVICE(d);
> > 
> >         [...]
> >         object_property_add_const_link(qdev_get_machine(), "smram",
> >                                        OBJECT(&mch->smram), &error_abort);
> >         [...]
> >     }
> 
> This is a link to a memory region.  A connection to a separate device
> can be found in hw/dma/xilinx_axidma.c and hw/net/xilinx_axienet.c,
> where you have
> 
>          data stream <------------> data stream
>        /                                        \
>    dma                                            enet
>        \                                        /
>          control stream <------> control stream
> 
> where the horizontal links in the middle are set up by board code, while
> the diagonal lines on the side are set up by device code.
> 
> > Paolo, can you provide guidance on when to use a bus, and when not to?
> 
> I would definitely use a bus if 1) it is common for the user (and not
> for machine code) to set up the connection 2) the relationship is
> parent-child.  Link properties are basically unused on the command line,
> and it only makes sense to make something different if the connection is
> some kind of graph so bus-child does not cut it.
> 

Definitely looks like the bus is the way to go. The controller/namespace
relationship is strictly parent-child.

Thanks both of you for the advice!


Klaus