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

[Gerd Hoffmann <kraxel@redhat.com>]

On Do, 2016-06-23 at 19:38 +0300, Michael S. Tsirkin wrote:
> 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.

Yep, we can implement the "soft-phys-bits" that way.

> bios doesn't want to parse that
> so it can go by some fw cfg file instead.

firmware can't use it anyway because the firmware first maps the bars,
the loads acpi tables (while qemu generates _CRS entries according to
the bios mappings).

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

Indeed, so I guess we'll have to stick to the current approach of
mapping 64bit bars above ram (or etc/reserved-memory-end if present).

cheers,
  Gerd