Re: [Qemu-devel] [RFC PATCH v6 0/6] Virtio refactoring.

[Anthony Liguori <aliguori@us.ibm.com>]

"Michael S. Tsirkin" <mst@redhat.com> writes:

> On Mon, Jan 07, 2013 at 02:12:23PM -0600, Anthony Liguori wrote:
>> "Michael S. Tsirkin" <mst@redhat.com> writes:
>> 
>> > On Tue, Dec 18, 2012 at 12:30:20PM +0100, KONRAD Frédéric wrote:
>> >> On 18/12/2012 12:01, Michael S. Tsirkin wrote:
>> >> >On Tue, Dec 18, 2012 at 10:33:37AM +0000, Peter Maydell wrote:
>> >> >>On 17 December 2012 15:45, Michael S. Tsirkin <mst@redhat.com> wrote:
>> >> >>>Is the point to allow virtio-mmio?  Why can't virtio-mmio be just
>> >> >>>another bus, like a pci bus, and another binding, like the virtio-pci
>> >> >>>binding?
>> >> >>(a) the current code is really not very nice because it's not
>> >> >>actually a proper set of QOM/qdev devices
>> >> >>(b) unlike PCI, you can't create sysbus devices on the
>> >> >>command line, because they don't correspond to a user
>> >> >>pluggable bit of hardware. We don't want users to have to know
>> >> >>an address and IRQ number for each virtio-mmio device (especially
>> >> >>since these are board specific); instead the board can create
>> >> >>and wire up transport devices wherever is suitable, and the
>> >> >>user just creates the backend (which is plugged into the virtio bus).
>> >> >>
>> >> >>-- PMM
>> >> >This is what I am saying: create your own bus and put
>> >> >your devices there. Allocate resources when you init
>> >> >a device.
>> >> >
>> >> >Instead you seem to want to expose a virtio device as two devices to
>> >> >user - if true this is not reasonable.
>> >> >
>> >> The modifications will be transparent to the user, as we will keep
>> >> virtio-x-pci devices.
>> >
>> > So there are three ways to add virtio pci devices now.
>> > Legacy -device virtio-net-pci, legacy legacy -net nic.model=virtio
>> > and the new one with two devices.
>> > If yes it's not transparent, it's user visible.
>> > Or did I misunderstand?
>> >
>> > Look we can have a virtio network device on a PCI bus.
>> > A very similar device can be created on XXX bus, and
>> > we can and do share a lot of code.
>> > This makes it two devices? Why not 4?
>> > One for TX one for RX one for control one for PCI.
>> > I hope I'm not giving anyone ideas ...
>> 
>> Devices != things users need to worry about.
>> 
>> The documented way to create network devices is completely different
>> than any possible syntax you can conjure up with -device.
>> 
>> Really, -device is not something users should have to deal with--ever.
>> It's a low level API, not a UI.
>> 
>> Regards,
>> 
>> Anthony Liguori
>
> Interesting.
> Let's assume I want to put a device behind a pci bridge
> (for example I want more than 32 of these).

You don't want to put a device behind a PCI bridge, you want to have
more than 32 devices.

'-net nic' should do the Right Thing when presented with more than 32
devices.

> It's impossible without -device, isn't it?

Think of -device like an API and -net as our UI.  Management tools want
to use an API, because it provides low level control and generally has
limited side effects.

Users want a UI that makes sense.  Trying to make both things satisfy
both audiences will almost certainly fail.

If a common use case cannot be done without resorting to using our API,
then we ought to improve our UI.

Regards,

Anthony Liguori

>
>> >
>> > -- 
>> > MST