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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 12:05:07PM +0100, Pawel Moll wrote: > On Wed, 2013-07-31 at 11:29 +0100, Michael S. Tsirkin wrote: > > Em, which one of specs? What I'm talking about is reserving a "Subsystem > > > Device ID" (p.2.1 of the current spec) value 0 to a "device that does > > > nothing" (is not a network card, nor block device, nor console, nor etc. > > > nor anything else) and specifying how is the guest supposed to behave > > > with such device (do nothing). In the current speech it would be > > > "Appendix I: Doing Nothing Device", following "Appendix I: SCSI Host > > > Device". How does this break anything? > > > > It doesn't but then it will have to keep doing nothing :) > > And that's the general plan. Confused. You wrote: usecase: create a pool of memory mapped devices and assign functions to them later Sounds like you really want a way to say "there's no device there" not "there's a device here but it does nothing". > I had a vague idea of "dynamic > devices" (ie. changing their "identity" in runtime), but this is just > one step too far. > > On Wed, 2013-07-31 at 11:34 +0100, Michael S. Tsirkin wrote: > > Reading through this, you want a way to say "there's no device there" > > for virtio-mmio? > > And maybe virtio-ccw. And virtio-smoke-signals is anyone ever implements > it :-) Yay, virtio-isa :). Many buses have device presence detection built-in. Many have hotplug support. If a bus doesn't have these you have to design something device specific but I don't think it makes sense to add this overhead for buses that do. > But seriously speaking, I'm just trying to formalize the current > behaviour of the core. Today it does nothing if there is no driver for a > particular VIRTIO_ID_*, and there is no (and will never be) driver with > 0 in id_table. The core might not do anything, but the OS does (e.g. prompt for a driver). Basically spec describes what Linux virtio does, but it's not possible to support what spec describes (match all device IDs, use subsystem ID to select the driver) on all OS-es. So we need to fix this part of spec, not build upon this bug to add more features that not all guests will be able to support. > > > > I think this is a bus-specific problem. PCI has a different, standard > > way to do this (report 0xfffffffff as both device and vendor ID). > > > And I want to provide a bus-agnostic solution. Of course the PCI > backends may never instantiate such device if they wish (and it probably > wouldn't make any sense whatsoever). > > Paweł > There's no way to implement this in a bus-agnostic way - it conflicts with existing bus-specific mechanisms. So just put this in the virtio-mmio appendix and don't try to anticipate the future. > > --------------------------------------------------------------------- > To unsubscribe from this mail list, you must leave the OASIS TC that > generates this mail. Follow this link to all your TCs in OASIS at: > https://www.oasis-open.org/apps/org/workgroup/portal/my_workgroups.php
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