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Date: Fri, 4 Oct 2002 23:20:12 -0400 (EDT)
From: Alexander Viro <viro@math.psu.edu>
To: Linus Torvalds <torvalds@transmeta.com>
cc: "David S. Miller" <davem@redhat.com>, linux-kernel@vger.kernel.org
Subject: Re: oops in bk pull (oct 03)
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On Fri, 4 Oct 2002, Linus Torvalds wrote:

> The more I think about this, the more convinced I am this is the case. We
> just _mustn't_ set up a live PCI window at address 0, and expect it to not
> cause confusion.
> 
> Also, we've seen before that we must not blindly disable a PCI window
> either, since that will kill the system when the host bridge is disabled
> and there is any pending DMA, for example (*). We saw that earlier in the
> 2.4.x tree - some host bridges will just ignore the disable (which means
> that then we'd trigger the zero-base bug), and others will honour the
> disable (which in turn will cause the DMA and other random problems).
> 
> This is all probably dependently on host bridge / MCH behaviour, so it
> probably works fine on 90%+ of all machines, but clearly breaks enough to
> not be a viable approach in general.

It's getting better.  The thing _does_ survive if there is no cacheline
boundary between the calls of pci_write_config_dword(); otherwise it
dies on that boundary.  So it depends not only on machine and compiler,
but on kernel config, and in a pretty random way (functions are aligned,
indeed, but not cacheline-aligned, so change of length in a function can
shift the rest of image relative to cachelines).

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