[SeaBIOS] [PATCH] acpi: hide 64-bit PCI hole for Windows XP

[Michael S. Tsirkin]

On Wed, Aug 07, 2013 at 09:29:39AM +0200, Gerd Hoffmann wrote:
>   Hi,
> 
> >> For whatever we don't know about, why should we assume 64-bit BARs
> >> work?  Especially considering it's likely to be pretty old guests.
> 
> > There's no need to assume 64 bit BARs works.
> > But I think we can assume guests don't crash.
> > What windows does here is very unusual imho,
> > guest should just say "I can't use this range
> > so I won't" and allocate BARs somewhere else.
> 
> Why windows crashes or what it should do doesn't matter.  We have to
> deal with it.
> 
> Today we have a pretty simple logic in seabios:  When we run out of
> space in the 32bit pci memory window we'll enable the 64bit window and
> try to move 64bit bars there.  So with a usual configuration there is no
> 64bit window simply because it isn't needed.  WinXP is happy.  If you
> plug in pci devices with large 64bit bars the 64bit window shows up.
> So, how about simply keeping that logic?
> We can drop the etc/pci-info logic then.

I don't think this is a good idea.

First, windows XP currently crashes if you supply a device with a
large BAR.
That's not a friendly way to tell user their OS
doesn't support the device, not enabling the device is better.

Second, current logic has *no chance* to support hotplug
of devices with large BARs. We will put work arounds for
XP in place to just disable 64 bit hole,
but for example with windows 7, 32 and 64 bit versions
both support a 64 bit hole but different max sizes,
and there's no easy way to distinguish between them.

Third, there are good reasons for pci-info, and
windows bugs don't outweight them.
In particular, there's duplicated logic in bios and QEMU,
if you change QEMU to be closer to real hardware, kaboom.

Last, it's a maintainance problem that we need
drivers for all chipsets open-coded in BIOS.
One suggestion we had is moving completely away
and supplying some bytecode for what's left of chipset
configuration (e.g. memcfg) to seabios.


> 
> We'll need some way for seabios to fill in the pci window information
> into the qemu-provided tables.  Easiest way to do that would be to
> extend the COMMAND_ADD_POINTER bios linker script command.
>
> We probably also need some knob asking seabios to enable the 64bit
> window unconditionally, so we have some address space for hotplugging
> pci devices with large bars.

I think you got it wrong. The common case is enabling
the 64 bit window. The default should be to enable it,
with a knob to disable.

We must also supply the 64 bit window size as
the limits differ between windows guests.
That's exactly the etc/pci-info logic.


> Comments?
> 
> cheers,
>   Gerd