Re: Possible issue with dangling PCI BARs

Benjamin Herrenschmidt
2007-12-13T03:40:26Z

On Thu, 2007-12-13 at 14:05 +1100, Benjamin Herrenschmidt wrote:
> On Thu, 2007-12-13 at 14:00 +1100, Benjamin Herrenschmidt wrote:
>
> .../...
>
> (oops, sent too fast)
>
> > So not only we can have a dangling BAR, but nothing prevent us to
> > actually go turn IO or MEM decoding on in case it wasn't already the
> > case on that device.
>
> And I was about to say before I clicked "send".. can't we do something like
> writing all ff's into the BAR at the same time as we clear res->start ? Isn't
> that supposed to pretty much disable decoding on that BAR ? Or not... Probably
> still better than leaving it to whatever dangling value it had no ?

Ok, reading some other threads, it seems that writing all ff's will not
be a very good alternative on x86 machines where MMCONFIG sits up
there...

I suppose there is nothing totally safe that can be done, thanks to
Intel not thinking about making BARs individually enable/disable'able
(or size-able without interrupting access, among other numerous fuckups
in the PCI spec).

So if a BAR is left dangling, I think we -must- disable MEM and IO
decoding on the whole device. In fact, the whole trick of passing a
bitmask of required BARs to pci_enable_device_bars() in the first place
doesn't fly.

Yuck.

Ben.