* Gerd Hoffmann (<email address hidden>) wrote:
> Hi,
> 
> >   d) For smaller amount of RAM it might still fail if
> > RAM+rounding+pci+hotplug space goes over the limit.
> >      Figuring that limit out is tricky (and I thought it
> >      might be BIOS/EFI dependent as well depending where they
> >      decide to put their PCI devices)
> 
> Both seabios and ovmf try to not go too high in address space.  Reason
> is exactly the phys-bits issue.  Using 40 here by default does not only
> limit the memory to 1TB.  It also has the problem that the guest thinks
> it has 1TB of address space but in reality it might be less.  Even
> recent skylake machines have phys-bits=39 (512G) only, and trying to use
> the physical address space above 512G in the guest just doesn't work
> because the phys-bits=39 limit applies to EPT too.
> 
> So checking phys-bits in the firmware, for example to place pci bars as
> high as possible in physical address space, is not going to work.
> 
> IIRC ovmf uses a 32G sized region with 32G alignment by default, which
> will land below 64G (aka phys-bits=36 address space) unless the guest
> has more than 30 (q35) or 31 (piix4) GB of memory.
> 
> seabios will not map pci bars above 4G unless it runs out of space below
> 4G.  If needed 64bit PCI bars will be placed right above ram, with
> gigabyte alignment.

Yep, I was tempted to set host-phys-bits=true on upstream, but TCG
has a fixed 40 bits last time I looked.

Dave

> cheers,
>   Gerd
> 
> -- 
> You received this bug notification because you are subscribed to the bug
> report.
> https://bugs.launchpad.net/bugs/1769053
> 
> Title:
>   Cannot start a guest with more than 1TB of RAM
> 
> Status in QEMU:
>   Incomplete
> Status in qemu package in Ubuntu:
>   Incomplete
> 
> Bug description:
>   Attempting to start a KVM guest with more than 1TB of RAM fails.
> 
>   It looks like we might need some extra patches:
>   https://lists.gnu.org/archive/html/qemu-discuss/2017-12/msg00005.html
> 
>   ProblemType: Bug
>   DistroRelease: Ubuntu 18.04
>   Package: qemu-system-x86 1:2.11+dfsg-1ubuntu7
>   ProcVersionSignature: Ubuntu 4.15.0-20.21-generic 4.15.17
>   Uname: Linux 4.15.0-20-generic x86_64
>   ApportVersion: 2.20.9-0ubuntu7
>   Architecture: amd64
>   CurrentDesktop: Unity:Unity7:ubuntu
>   Date: Fri May  4 16:21:14 2018
>   InstallationDate: Installed on 2017-04-05 (393 days ago)
>   InstallationMedia: Ubuntu 16.10 "Yakkety Yak" - Release amd64 (20161012.2)
>   MachineType: Dell Inc. XPS 13 9360
>   ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-4.15.0-20-generic root=/dev/mapper/ubuntu--vg-root ro quiet splash transparent_hugepage=madvise vt.handoff=1
>   SourcePackage: qemu
>   UpgradeStatus: Upgraded to bionic on 2018-04-30 (3 days ago)
>   dmi.bios.date: 02/26/2018
>   dmi.bios.vendor: Dell Inc.
>   dmi.bios.version: 2.6.2
>   dmi.board.name: 0PF86Y
>   dmi.board.vendor: Dell Inc.
>   dmi.board.version: A00
>   dmi.chassis.type: 9
>   dmi.chassis.vendor: Dell Inc.
>   dmi.modalias: dmi:bvnDellInc.:bvr2.6.2:bd02/26/2018:svnDellInc.:pnXPS139360:pvr:rvnDellInc.:rn0PF86Y:rvrA00:cvnDellInc.:ct9:cvr:
>   dmi.product.family: XPS
>   dmi.product.name: XPS 13 9360
>   dmi.sys.vendor: Dell Inc.
> 
> To manage notifications about this bug go to:
> https://bugs.launchpad.net/qemu/+bug/1769053/+subscriptions
--
Dr. David Alan Gilbert / <email address hidden> / Manchester, UK