Re: [Fwd: Re: [PATCH 0/5]PCI: x86 MMCONFIG]

[Tony Camuso <tcamuso@redhat.com>]

Matthew Wilcox wrote:

> Here's how BARs work ... when you write 0xffffffff to the BAR, it
> ignores all the set bits that are less than the size of the BAR.  So,
> assuming this is a 256MB BAR (like my G33 is), what ends up written to
> this BAR is 0xf0000000.  Now, because this is graphics, apparently it's
> special and embedded in the chipset, even though it looks like it's a
> PCI device.  So it actually gets priority over MMCONFIG which is also
> mapped to 0xf0000000.
> 
> For your case of a 64-bit BAR, you could write 0xffffffff to the high
> 32-bits first, then write to the low 32-bits, then reset the low, then
> high bits, and you'd avoid the problem.  But the G33 has a 32-bit BAR
> with the same problem, so it won't work for that case.
> 
> BARs that are behind bridges don't have this problem (they can't decode
> memory accesses that aren't forwarded to them).  BARs on devices which
> have memory IO disabled also don't have theis problem, but disabling
> devices has its problems (as does probing BARs for active devices anyway
> ...).
>
Thanks for the detailed explanation.

 > The question is how large can 32-bit BARs get.  As we've seen, 256MB
 > exist, and are causing pain.  I can't imagine any PCI device
 > manufacturer thinks they can allocate 2GB of the low space, but we could
 > potentially mis-size a large BAR by not using 0xffffffff.
 >

Point well taken. Graphics devices understandably consume a lot of memory
space, and are likely to consume even more in the not-too-distant future.

> I'm really not clear on the purpose of your patchset.  Was it all to
> address this one problem?
> 

No. My patch-set does not address this problem at all, but rather the
larger problem of having mmconfig-unfriendly devices on buses that are
out of reach of the unreachable_devices() routine and bitmap.

This problem is one I encountered during my testing and mentioned in
my preamble as not being fixable by my patch-set.