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Subject: Re: [virtio] Groups - Action Item "Create text version of virtio 0.9.5 document" added
Pawel Moll <pawel.moll@arm.com> writes: > On Wed, 2013-07-31 at 14:25 +0100, Anthony Liguori wrote: >> I'm not sure where this discussion took place, but there is nothing in >> QEMU that requires a dummy device to be used. > > http://thread.gmane.org/gmane.linux.kernel.virtualization/20144 N.B. If you aren't discussing something on qemu-devel, then you aren't getting the full input of the community. >> The analogy would be a PCI slot that doesn't have a device plugged into >> it. Per the PCI spec, if a device is absent, any reads to the slot >> will return a well defined value (all ones). Those values are invalid >> for things like a device id. > > It is very unsafe to "probe" memory mapped peripherals. It simply depends on the devices. Prior to PnP, probing was common practice on the PC. Yes, sometimes badness ensued. >> I really dislike the design of virtio-mmio. I don't understand why it >> limits itself to a single device. > > It is modelled after simple memory mapped peripherals, so it is simple > itself. And if I was doing it today from scratch again, I would still > focus on doing one thing only. After all there are discoverable buses > out there, I don't see any reason to try to invent my own discovery > mechanism. Right, I've already argued with Peter that ya'll should simply use PCI for mach-virt... We don't need to repeat that here though. >> virtio-blk is a separate device that can be connected to the transport. > > As in: block device of zero size? This has also been suggested in the > thread linked above. That's irrelevant here. >> And yes, there should be a way to specify "this transport is not >> connected to anything". > > In a "virtio generic" way or mmio-specific one? If we can do it in a backwards compatible fashion, I think it would be a very good idea to reserve a device ID to indicate that there is not a valid device connected to the transport. > Maybe instead of > defining zero as a do-nothing-device we should simply make this value > reserved or illegal? Yes, I think that's a good idea. > This would make PCI situation clear (no device will > be ever allowed to have it) and mmio driver could "overload" its meaning > as "ignore". Agreed. Regards, Anthony Liguori > > Paweł
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