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Subject: Re: [virtio] Groups - Action Item "Create text version of virtio 0.9.5 document" added
On Wed, Jul 31, 2013 at 03:37:01PM +0100, Pawel Moll wrote: > 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 > > > 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. > > > 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. > > > But that's a different discussion. > > It is indeed, and I am open to all suggestions how to make it more > likeable :-) > > > 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. > > > 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? Maybe instead of > defining zero as a do-nothing-device we should simply make this value > reserved or illegal? This would make PCI situation clear (no device will > be ever allowed to have it) and mmio driver could "overload" its meaning > as "ignore". > > Paweł This last option is what I was suggesting, I think it'll work well.