linux-kernel - Re: Possible issue with dangling PCI BARs

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Message-Id: <1197517200.15741.68.camel@pasglop>
Date:	Thu, 13 Dec 2007 14:40:00 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	linux-pci@...ey.karlin.mff.cuni.cz
Cc:	Linux Kernel list <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: Possible issue with dangling PCI BARs

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.

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