* ChristianEhrhardt (<email address hidden>) wrote:
> > 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.

I assume so, but I've not looked at the detail of that.

> > 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.
> 
> Defaults:
> 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

OK, good - that suggests there's nothing missing.
We enable host-phys-bits=true by default I think (in our machine type?)

> 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.

The problem is there are a whole bunch of things that are hard to deal
with:
  a) Cheaper CPUs tend to have smaller phys-bits even in the same
generation; e.g. my laptop is still 36 bits, a lot are 39 bits.  I think
the same is true of the Xeon E3-.... family.   It makes it hard to know
what to pick when you're going to allow migration.

  b) Reasoning about the total address size range is difficult; you've
got to take into account PCI address space and hot plug space etc
to know where the upper edge is.

Dave

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