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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.

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