Re: [Qemu-devel] Default for phys-addr-bits? (was Re: [PATCH 4/5] x86: Allow physical address bits to be set)

[Michael S. Tsirkin]

On Thu, Jun 23, 2016 at 10:40:03AM +0200, Gerd Hoffmann wrote:
>   Hi,
> 
> > > Well the crash of guest phys bits > host phys bits, should be easy to
> > > reproduce by booting a 65GB guest on a 64GB RAM + 2GB swap host with
> > > 36 host phys bits using the upstream qemu that forces the guest phys
> > > bits to 40.
> > 
> > So you supply more RAM than host can address, and guest crashes?
> 
> Yep.  The only reason we don't see this happening in practice is that
> it's probably next to impossible to find a machine which has (a) only 36
> physical address lines and (b) allows to plug that much RAM.
> 
> > Why are we worried about it?
> 
> It's more a issue with pci ressources.  In theory seabios/edk2 could go
> figure how big the physical address space is, then map 64bit pci bars as
> high as possible, thereby making stuff like etc/reserved-memory-end in
> fw_cfg unnecessary.
> 
> But with qemu saying 40 phys bits are available even if they are not
> this approach isn't going to fly ...
> 
> cheers,
>   Gerd

Nah, x86 guests really need to go by _CRS. bios doesn't want to parse that
so it can go by some fw cfg file instead.

Going by phys bits won't work on old qemu so I don't believe it's
practical.

-- 
MST