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[Qemu-devel] [Bug 1769053] Re: Cannot start a guest with more than 1TB o

From: ChristianEhrhardt
Subject: [Qemu-devel] [Bug 1769053] Re: Cannot start a guest with more than 1TB of RAM
Date: Mon, 07 May 2018 10:33:34 -0000

> Interesting; I thought this was supposed to work.

Exactly that was my thought when triaging it initially
Furthermore I assume people working la57 (https://lwn.net/Articles/730925/) and 
such ran tests on much bigger sizes.

> Ah right Dan, if you're seeing the 40 bits physical in the guest you
definitely need to try the flags I suggest in comment 6; host-phys-
bits=true should work for you.

I tested Bionic to be at least on libvirt 4.0 / qemu 2.11.1 when we want
to check things under the "supposed to work now" flag.

Host:  address sizes   : 46 bits physical, 48 bits virtual
Guest: address sizes   : 40 bits physical, 48 bits virtual

I ensured that with option -cpu host,host-phys-bits=true set I successfully get 
what my host can provide in the guest:
Guest: address sizes   : 46 bits physical, 48 bits virtual

Starting a guest with that >1TB (that would be mostly on swap if needed) works 
just fine as expected. Here ~1063 GB from /proc/meminfo
MemTotal:       1114676492 kB

I also checked a more compatible approach like -cpu qemu64,phys-bits=42
and that works as well.

IMHO - if anything - one could argue that libvirt/qemu could be smarter
about e.g. auto adding those arguments (or print a warning) when
crossing a certain memory size.

So for now I'd stick to the "actually works" summary and keep the status
to incomplete.

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  Cannot start a guest with more than 1TB of RAM

Status in QEMU:
Status in qemu package in Ubuntu:

Bug description:
  Attempting to start a KVM guest with more than 1TB of RAM fails.

  It looks like we might need some extra patches:

  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 
  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.product.family: XPS
  dmi.product.name: XPS 13 9360
  dmi.sys.vendor: Dell Inc.

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