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

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Jun 24, 2016 at 07:55:33AM +0200, Gerd Hoffmann wrote:
> 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.

The advantage being no guest changes are necessary. Guests
already use _CRS.

> > 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