[Bug 1769053]

[raistlin <1769053@bugs.launchpad.net>]

Hello.

Recently I had to deal with a VM with ~2.7 TB of RAM. The [open]SUSE
QEMU package carries a patch for bumping the default maximum virtual
address bits to 42 (from 40). Now, the last entry of the VM's e820 was
this one:

    BIOS-e820: [mem 0x0000000100000000-0x000002b57fffffff] usable

Which, if I have computed correctly, is representable on 42 bits, so
things should be fine. However, during boot, the VM shows this:

    L1TF: System has more than MAX_PA/2 memory. L1TF mitigation not
effective

And if I look in /sys/devices/system/cpu/vulnerabilities/l1tf, I see
this:

    l1tf: Vulnerable

This is because, while the RAM fits in MAX_PA=42, as soon as we take 1
bit off for PTE inversion, it does not fit any longer (in MAX_PA/2).

I understand that this is not critical per-se, but I think it's rather
annoying for a user to see messages like the ones above, especially
considering they're about vulnerabilities and security. And it's not
necessarily easy for everyone to realize that L1TF is reported as
vulnerable because QEMU is making the VM think that physical addresses
are on 42 (or 40) bits.

So, I also think we need to be able to tweak this part of the VM
configuration more easily, from libvirt. It's doable either by using
specially modified CPU-models, or doing things like this, which are
rather inconvenient:

    <qemu:commandline>
      <qemu:arg value='-cpu'/>
      <qemu:arg value='host,host-phys-bits=on'/>
    </qemu:commandline>

I also believe that host-phys-bits=on should be QEMU's default when the
user chooses host as CPU model, but that's for another bugzilla. :-)

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https://bugs.launchpad.net/bugs/1769053

Title:
  Ability to control phys-bits through libvirt

Status in libvirt:
  Confirmed
Status in QEMU:
  Invalid
Status in libvirt package in Ubuntu:
  Triaged
Status in qemu package in Ubuntu:
  Invalid

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.

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