On 22.07.2013, at 21:38, Anthony Liguori wrote: > Alexander Graf <ag...@suse.de> writes: > >> Platforms without ISA and/or PCI have had a seriously hard time in the >> dynamic >> device creation world of QEMU. Devices on these were modeled as SysBus >> devices >> which can only be instantiated in machine files, not through -device. >> >> Why is that so? >> >> Well, SysBus is trying to be incredibly generic. It allows you to plug any >> interrupt sender into any other interrupt receiver. It allows you to map >> a device's memory regions into any other random memory region. All of that >> only works from C code. >> >> But do we need that level of complexity for normal devices usually? In a >> normal platform world (SoCs, PV machines) we have a flat memory layout we >> can plug our device memory into. We also have a flat IRQ model where we >> can plug our device IRQs into. >> >> This platform bus creates a simple bus that models the easy world. It allows >> for dynamic device creation through -device. A device may or may not >> explictly >> request to be mapped at a specific IRQ and/or memory address. If no explicit >> mapping is requested, platform devices just get mapped at convenient places. >> >> This goes hand in hand with automatic device tree generation. When QEMU >> places devices somewhere and also generates a device tree to tell the guest >> where exactly those devices are, we close the cycle and everyone knows >> everything and lives happily ever after. >> >> The actual pressing issue this solves is that today it's impossible to spawn >> serial ports from the command line. With this patch set, it's possible to >> do so. But it lays the groundwork for much more... > > tl;dr, this is a PV bus for the e500 described as something more > generic. I don't buy it. I don't think there are many platforms or > devices out there where you can arbitrarily hook up devices at random > offsets/IRQs and expect things to work in any meaningful way. > > So I'll suggest one of three things: > > 1) Just use PCI and call it a day
If only the world was this easy. We already use PCI for everything where it makes sense today, but some devices I want to create dynamically from the command line simply aren't on PCI. > 2) Rename this to E500PlatformBus and call it a day This could be used just the same for ARM's mach-virt, so I'd rather not make it e500 specific. > 3) If you really want to solve the general problem of platform devices, > I think the approach needs to involve: > > a) Forget about any bus at all, just inherit from a common base class > > b) Use the common base class to expose interfaces to enumerate IRQs > and MemoryRegions > > c) Have machine-specific logic (in the post init hook) that looks for > any device of the type defined in (a) and connects their > IRQs/MemoryRegions as appropriate. Ok, I have something very similar for the device tree walker anyways, just that I'm walking a bus instead of the global device pool. But the idea is probably still the same. I suppose I'd still inherit from PlatBusDevice (which obviously would be called PlatformDevice or SimpleDevice by then) to get me my interfaces, right? I'll see what I can come up with. Alex
