QEMU Virtual Machine Manager L3 cache sharing on a 7950x3d

[Witnessthis]

Hi, I'm currently virtualizing windows 11 in linux using qemu and Virtual Machine Manager. I'm currently passing through physical cores 0-7 which are sharing the same L3 cache and I'm trying to configure the cpu layout to be the same(ish) as that of my 7950x3d:

 

 

What I'm getting on the guest however is  all logical cpus detected as separate cores each with a 16MB L3 cache.

I've been trying to mess around with the cputune configuration and I, first of all, can't seem to achieve having two cpus per core like in the above picture, but I also can't seem to have them share L3 cache.

I have tried using the cachetune parameter to categorize my cores into different L3 groups, but when I start the VM I get an error message saying that "Resource control is not supported on this host". I also get the same error message when trying to add the cache mode="passthrough" to the cpu configuration option.

So my question is whether ryzen 7950x3d even supports this and if not, what specification is needed on a cpu to know if it supports this use case? As in what do I look for when I buy a CPU with these capabilities?

Thanks in advance!

[Witnessthis]

I believe I solved this myself now!!! Also I found a bug in my configuration from before. The assumption that <cache mode="passthrough"/> generated the same error message for me was false. I had incorrectly left the cachetune parameter in the configuration when I tested it.

 

The missing piece of configuration seemed to be in the cpu section.

I had to add two lines to the configuration:

 

<cpu mode="host-passthrough" check="none" migratable="on">

  <topology sockets="1" dies="1" cores="8" threads="2"/>

  <cache mode="passthrough"/>

  <feature policy="require" name="topoext"/>

</cpu>

However, I still don't know what would be needed to get the cputune configuration to work properly.

Anyway, problem solved for now! :)