[Qemu-devel] Re: [PATCH V6 17/32] pci: 64bit bar support.

[Isaku Yamahata]

On Sun, Nov 01, 2009 at 06:07:30PM +0200, Michael S. Tsirkin wrote:
> On Fri, Oct 30, 2009 at 09:21:11PM +0900, Isaku Yamahata wrote:
> > implemented pci 64bit bar support.
> > The tricky bit is pci_update_mapping().
> > An OS is allowed to set the BAR such that OS can't address the area
> > pointed by BAR. It doesn't make sense, though.
> 
> It might make sense. 32 bit guest can address more than 4G of
> physical RAM, e.g. using PAE.

Yes, in that case, guest OS will set bar to be under 36 bit.
If PAE were supported, target phys address would be 64 bit.


> Since I think qemu can not support this if target phys address is 32
> bit, we should declare lack of support for 64 bit addressing on these
> platforms, by forcing BAR into 32 bit mode, rather than silently failing
> to map it.

I don't get your point. And I don't understand the benefit of focing
BAR into 32 bit mode.
Real hardware silently maps BAR to address beyond CPU addressable
area.
Let's stick to PCI spec as you said before.

32 bit guest OS will set 64 bit BAR to be smaller 32bit
(or 36bit if PAE). That's it.
Even if 64 bit CPU case, architectally addressable address bit is smaller
than 64 bit. It's CPU implementation dependent.
And guest OS sets BAR according to it.

-- 
yamahata