Re: [Qemu-devel] [SeaBIOS] [PATCH] hw/pci: reserve IO and mem for pci-2-pci bridges with no devices attached

[Marcel Apfelbaum <marcel.a@redhat.com>]

On Mon, 2014-04-07 at 16:34 +0300, Michael S. Tsirkin wrote:
> On Mon, Apr 07, 2014 at 02:44:06PM +0200, Gerd Hoffmann wrote:
> >   Hi,
> > 
> > > > +        u8 shpc_cap = pci_find_capability(s->bus_dev, PCI_CAP_ID_SHPC);
> > 
> > > One thing I'd do is maybe check that the relevant memory type is
> > > enabled in the bridge (probably just by writing fff to base and reading
> > > it back).
> > 
> > > This will give hypervisors an option to avoid wasting resources:
> > > e.g. it's uncommon for express devices to claim IO.
> > 
> > I don't think we'll need that for the SHPC bridge.
> 
> Why not?
Because "has shpc" => not an PCIe port. (as far as I know)
Anyway, why have shpc capability but no I/O or mem to support it?

Thanks,
Marcel

> I'm referring to this text in the bridge specification:
> 
> 	The I/O Base and I/O Limit registers are optional and define an address
> 	range that is used
> 	by the bridge to determine when to forward I/O transactions from one
> 	interface to the
> 	other.
> 	If a bridge does not implement an I/O address range, then both the I/O
> 	Base and I/O
> 	Limit registers must be implemented as read-only registers that return
> 	zero when read. If a
> 	bridge supports an I/O address range, then these registers must be
> 	initialized by
> 	configuration software so default states are not specified.
> 
> So we should probe bridge for I/O support before wasting I/O resources on it.
> The spec does not provide a way to detect this, but we can do it like this:
> 
> 	- write value ffffffff to I/O base register
Why write? A simple read would be enough.
It will never be 0(if I/O or mem is required) because of the "Base Address"
part of the register which represents the address range, right?

Thanks,
Marcel

> 	- read back value
> 
> value 0 means bridge does not support I/O.
> 
> 
> A similar trick should work for other optional resources.
> 
> 
> > For express it indeed makes sense to avoid claiming IO address space.
> > I'd try to find something more automatic though, where you don't need
> > some kind of "disable io for this express port" config option.
> 
> Won't same trick as above work?
> 
> > For express ports which can only have a single device underneath we can
> > check whenever we have a device and if one is present already don't
> > bother claiming extra resources for hotplug.
> > 
> > > > +    for (cap = pci_config_readb(pci->bdf, PCI_CAPABILITY_LIST); cap;
> > > > +                cap = pci_config_readb(pci->bdf, cap + PCI_CAP_LIST_NEXT))
> > > > +        if (pci_config_readb(pci->bdf, cap + PCI_CAP_LIST_ID) == cap_id)
> > > > +            return cap;
> > > 
> > > I would also limit this to 256 iterations, to make sure
> > > we dont' get into an infinite loop with a broken device.
> > 
> > Good point.
> > 
> > cheers,
> >   Gerd
> >