kvm PCI assignment & VFIO ramblings

[Roedel, Joerg]

On Mon, Aug 22, 2011 at 05:03:53PM -0400, Benjamin Herrenschmidt wrote:
> 
> > I am in favour of /dev/vfio/$GROUP. If multiple devices should be
> > assigned to a guest, there can also be an ioctl to bind a group to an
> > address-space of another group (certainly needs some care to not allow
> > that both groups belong to different processes).
> > 
> > Btw, a problem we havn't talked about yet entirely is
> > driver-deassignment. User space can decide to de-assign the device from
> > vfio while a fd is open on it. With PCI there is no way to let this fail
> > (the .release function returns void last time i checked). Is this a
> > problem, and yes, how we handle that?
> 
> We can treat it as a hard unplug (like a cardbus gone away).
> 
> IE. Dispose of the direct mappings (switch to MMIO emulation) and return
> all ff's from reads (& ignore writes).
> 
> Then send an unplug event via whatever mechanism the platform provides
> (ACPI hotplug controller on x86 for example, we haven't quite sorted out
> what to do on power for hotplug yet).

Hmm, good idea. But as far as I know the hotplug-event needs to be in
the guest _before_ the device is actually unplugged (so that the guest
can unbind its driver first). That somehow brings back the sleep-idea
and the timeout in the .release function.

	Joerg

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