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

[Gerd Hoffmann <kraxel@redhat.com>]

On 12/16/11 05:41, Alexey Korolev wrote:
>>> 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?

Yes.  There is work in progress to add pci-express and a more modern
chipset emulation to qemu.

I think for now it is fine if we simply map everything behind a bridge
below 4G and hash out the details (i.e. whenever we can move the bridge
memory window above 4G or not) later.

> I did some preliminary tests: 64bit BARs
> are working quite well for linux 2.6.18 - 3.0 and windows 2008.

Nice.

> 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.

Agree.

> 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.

Yes, devices attached to the root bus don't have to care about bridge
windows.

cheers,
  Gerd