Re: MemoryBacking options

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[Once again, I ask you to keep the list on the CC. In gmail's web UI
it's the 'Reply to all' button].

On 10/22/22 11:19, Simon Fairweather wrote:
> Is there a way to limit the shared memory allocated via Libvirt to
> memory backing?

Yes, there is.
On the top level there's /domain/memoryBacking [1] that serves as the
default value for the guest. Then, under /domain/cpu/numa/cell [2]
individual guest NUMA nodes are configured and each can have @memAccess
attribute which overrides the top level setting (=memoryBacking). And
finally, under /domain/devices/memory [3] individual RAM modules can be
configured, and each one can have @access attribute which overrides both
top level AND guest NUMA node setting.

For instance:

  <maxMemory slots='16' unit='GiB'>8</maxMemory>
  <memory unit='GiB'>5</memory>
    <access mode='private'/>
      <cell id='0' memory='2' unit='GiB' memAccess='shared'/>
      <cell id='1' memory='2' unit='GiB' />
    <memory model='dimm' access='private'>
        <size unit='GiB'>1</size>

Here, the guest NUMA node 0 is going to be shared, except for DIMM
module attached to it (which is going to be private) and NUMA node 1 is
going to be private again.



> >
> On Mon, Oct 17, 2022 at 9:06 AM Michal Prívozník <mprivozn@xxxxxxxxxx
> <mailto:mprivozn@xxxxxxxxxx>> wrote:
>     [please keep the list on CC for benefit of the community]
>     On 10/16/22 09:58, Simon Fairweather wrote:
>     > Thanks Michal, Are there any known issues with Memory Backing. We have
>     > QEMU 7.1 and libvirt 8.7.
>     >
>     > If just Memory Backing is set the VM Freezes after a couple of
>     hours and
>     > the allocated CPUs show 100% on the host.
>     >
>     > Seems to happen more on AMD Platforms than Intel.
>     >
>     > Please advise on any info you need, likely this is a QEMU issue.
>     >
>     >   <memoryBacking>
>     >     <source type='memfd'/>
>     >     <access mode='shared'/>
>     >   </memoryBacking>
>     Agreed, this smells like QEMU issue. There's nothing obviously wrong
>     with this snipped. I have a VM with just that and run it just fine.
>     Although, I run it for couple of hours max.
>     Michal

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