Re: [Xen-devel] [PATCH, RFC]: qemu: hang-free/error-tolerant PCI hot-plug protocol

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Fri, 30 Jul 2010, Ian Jackson wrote:
> Gianni Tedesco writes ("[Xen-devel] [PATCH, RFC]: qemu: 
> hang-free/error-tolerant PCI hot-plug protocol"):
> > The interface for PCI hotplug is flexible enough to shoot ones-self in
> > the foot. It is possible to try to insert a PCI device in to a slot
> > already occupied by a qemu emulated device (NIC, PIIX, ISA-bridge, etc.)
> > In this case qemu (wisely) refuses to do the hotplug. Since there is no
> > way for a toolstack to query qemu's pci device layout there is no way to
> > check for this ahead of time. In this case the toolstack must wait for
> > device-model state to change to pci-inserted which never happens.
> 
> Hrm.
> 
> > I propose that when qemu decides not to hot-plug a device that it raise
> > the "pci-inserted" status anyway. The tools must then check the
> > "parameter" key in xenbus for a non-error string. In other words:
> 
> Why do this rather than a new status "pci-insert-failed" ?  How does
> this affect existing toolstacks ?
> 

A new return status would cause existing toolstacks to keep waiting
until the timeout expires.
On the other hand Gianni's approach is backward compatible.

> > --- a/hw/piix4acpi.c
> > +++ b/hw/piix4acpi.c
> 
> I haven't looked at the code near here.  Does qemu log anything ?  If
> so then the corresponding toolstack patches should say "consult qemu
> logfile".  Otherwise perhaps qemu should.

Consulting logfiles in case of errors is always a good idea, but we
should be able to detect that there was an error first, without waiting
for a timeout to expire.


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