On Thu, Apr 13, 2017 at 05:58:51PM +0100, Peter Maydell wrote: > On 13 April 2017 at 17:44, Eduardo Habkost <ehabk...@redhat.com> wrote: > > On Thu, Apr 13, 2017 at 06:15:34PM +0300, Roman Kagan wrote: > >> Can you (or anybody else) please help me decide if I need > >> TYPE_SYS_BUS_DEVICE? Logically the VMBus bridge is "attached directly > >> to the main system bus" as written at the top of include/hw/sysbus.h. > > > > I think that documentation was written before we supported > > bus-less devices. > > The major question to ask to determine whether you need to be > a sysbus device is how does the guest interact with this thing? > In particular, does it have memory mapped registers (and is it > not part of some other more specific interface like a PCI device) ?
The interaction happens through a set of dedicated MSRs, shared pages, hypercalls, and "synthetic" interrupts (non-ioapic). No pio, mmio, gpio/irq. VMBus annonces its presence by a device entry in ACPI with two IRQs; we've seen no need to wire those IRQs so far, and the guests don't seem to use them. That's it; everything else is VMBus-specific. > > I'm not sure about the consequences of simply connecting IRQs > > inside ->realize() without using the sysbus *_irq helpers. I hope > > others can clarify this. > > Connecting what IRQs to what? If your device has an IRQ > then it isn't the device's job to connect it up -- it is > the job of the machine model, because only the machine > model knows what the IRQ controller is, whether the IRQ > needs to be advertised via ACPI or device tree, and so on. > You can do that without sysbus though, by using the core > DeviceState's qdev_init_gpio_* APIs. (NB that if you do > this then you're by necessity not a device creatable > with -device, since the board code has to wire you up.) > > If you're not using sysbus then watch out for reset: > all sysbus devices get automatically reset on QEMU > system reset; if you're directly using the DeviceState > baseclass then you have to arrange reset for yourself > somehow. Thanks! Roman.
