Re: [Qemu-devel] [BUG] [Seabios] PCI 64bit BARs on Win2008 - unable to start the device. (ACPI lacks the _DSM method)

[Alexey Korolev <alexey.korolev@endace.com>]

>> I wonder if there any particular reason to separate prefetchable a
>> non-prefetchable memory regions in pciinit? Extra two more regions would
>> make code more complex.
> Oh yes, there is.  Which reminds me that the whole thing isn't that easy
> unfortunaly ...
>
> The reason are pci bridges.  They have two memory regions, one for
> prefetchable and one for non-prefetchable memory.  All devices behind a
> pci bridge must have the bars within the bridges memory regions, thats
> why they are grouped together.
>
> This also implies that a 32bit and a 64bit bar (of the same type) behind
> a pci bridge must be mapped next to each other, so moving up 64bit bars
> unconditionally isn't going to fly.  Devices behind bridges need some
> extra care, only when all bars are 64bit capable they can actually be
> mapped above 4G.
The qemu actually does not simulate PCI bridges at all. So good question 
shall we bother about this?  I did some preliminary tests: 64bit BARs 
are working quite well for linux 2.6.18 - 3.0 and windows 2008.

Also I've found important detail that according to PCI architecture 
specification, the bridges can describe 64bit ranges for prefetchable 
type of memory only. So it's very unlikely that devices exporting 64bit 
non-prefetchable BARs. I guess we just need to add one extra type.

Even if there are two separate prefetchable memory regions (32 bit and 
64bit), it won't be a problem as there is no bridge on the 440FX inside 
the virtual machine.

Cheers,
Alexey