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

[Markus Armbruster <armbru@redhat.com>]

Cc: QOM maintainers in case I'm talking nonsense about QOM.

Klaus Birkelund <klaus@birkelund.eu> writes:

> On Tue, Jun 25, 2019 at 07:51:29AM +0200, Markus Armbruster wrote:
>> Laszlo Ersek <lersek@redhat.com> writes:
>> 
>> > On 06/24/19 12:18, Kevin Wolf wrote:
>> >> Am 24.06.2019 um 10:01 hat Klaus Birkelund geschrieben:
>> >>> On Thu, Jun 20, 2019 at 05:37:24PM +0200, Laszlo Ersek wrote:
>> >>>> On 06/17/19 10:12, Klaus Birkelund wrote:
>> >>>>> Hi all,
>> >>>>>
>> >>>>> I'm thinking about how to support multiple namespaces in the NVMe
>> >>>>> device. My first idea was to add a "namespaces" property array to the
>> >>>>> device that references blockdevs, but as Laszlo writes below, this might
>> >>>>> not be the best idea. It also makes it troublesome to add per-namespace
>> >>>>> parameters (which is something I will be required to do for other
>> >>>>> reasons). Some of you might remember my first attempt at this that
>> >>>>> included adding a new block driver (derived from raw) that could be
>> >>>>> given certain parameters that would then be stored in the image. But I
>> >>>>> understand that this is a no-go, and I can see why.
>> >>>>>
>> >>>>> I guess the optimal way would be such that the parameters was something
>> >>>>> like:
>> >>>>>
>> >>>>>    -blockdev raw,node-name=blk_ns1,file.driver=file,file.filename=blk_ns1.img
>> >>>>>    -blockdev raw,node-name=blk_ns2,file.driver=file,file.filename=blk_ns2.img
>> >>>>>    -device nvme-ns,drive=blk_ns1,ns-specific-options (nsfeat,mc,dlfeat)...
>> >>>>>    -device nvme-ns,drive=blk_ns2,...
>> >>>>>    -device nvme,...
>> >>>>>
>> >>>>> My question is how to state the parent/child relationship between the
>> >>>>> nvme and nvme-ns devices. I've been looking at how ide and virtio does
>> >>>>> this, and maybe a "bus" is the right way to go?
>> >>>>
>> >>>> I've added Markus to the address list, because of this question. No
>> >>>> other (new) comments from me on the thread starter at this time, just
>> >>>> keeping the full context.
>> >>>>
>> >>>
>> >>> Hi all,
>> >>>
>> >>> I've succesfully implemented this by introducing a new 'nvme-ns' device
>> >>> model. The nvme device creates a bus named from the device id ('id'
>> >>> parameter) and the nvme-ns devices are then registered on this.
>> >>>
>> >>> This results in an nvme device being creates like this (two namespaces
>> >>> example):
>> >>>
>> >>>   -drive file=nvme0n1.img,if=none,id=disk1
>> >>>   -drive file=nvme0n2.img,if=none,id=disk2
>> >>>   -device nvme,serial=deadbeef,id=nvme0
>> >>>   -device nvme-ns,drive=disk1,bus=nvme0,nsid=1
>> >>>   -device nvme-ns,drive=disk2,bus=nvme0,nsid=2
>> >>>
>> >>> How does that look as a way forward?
>> >> 
>> >> This looks very similar to what other devices do (one bus controller
>> >> that has multiple devices on its but), so I like it.
>> 
>> Devices can be wired together without a bus intermediary.  You
>> definitely want a bus when the physical connection you model has one.
>> If not, a bus may be useful anyway, say because it provides a convenient
>> way to encapsulate the connection model, or to support -device bus=...
>> 
>  
> 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);
        [...]
    }

Paolo, can you provide guidance on when to use a bus, and when not to?