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

[Michael S. Tsirkin]

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?
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
        - 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
>