Re: [Qemu-devel] [RFC] map 64-bit PCI devices after all possible RAM

[Michael S. Tsirkin]

On Thu, Oct 10, 2013 at 02:14:16PM +0200, Gerd Hoffmann wrote:
>   Hi,
> 
> > I think the simplest way to do all this is simply to tell seabios
> > that we have more memory. seabios already programs 64 bit BARs
> > higher than memory.
> 
> Hmm?  As I understand Igor just wants some address space for memory
> hotplug.  So there wouldn't be memory there (yet).  And telling seabios
> there is although there isn't will make seabios place wrong info into
> the e820 tables.  Not going to fly.

True. Maybe we should get some smbios stuff from qemu too.

> > I think the issue is with legacy guests.
> > E.g. if VCPU claims to support 50 bit of memory
> > do we put high PCI memory at 1 << 50?
> > If yes old guests which expect at most 40 bit
> > will not be able to use it.
> 
> Hmm.  Sure such guests exist?

I wouldn't be surprised. At least some windows
guests crash if you try to tell them your system
has too much physical memory (e.g. 2^48).

>  Note this is physical address lines, not
> virtual address space (where you might need an additional level of
> pagetables to fully use it, which is not something we could expect old
> guests being able to handle).
> 
> cheers,
>   Gerd
>