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I have an Ubuntu host with an AMD Matisse processor (12C 24T). When launching virtual machines using libvirt (QEMU+KVM), there's the option to set CPU topology.

Almost all consumer PCs have single-socket CPU with multiple cores and up to 2 threads per core (HT), so I'm tempted to set a similar topology for the VM. I wonder if this setting alone (assume total CPU threads remains constant) affects VM performance or host CPU utilization, for example with regards to scheduling.

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    In the past consumer or home editions of Windows supported (I believe) only 1 socket so this might be a way to "convert" a multi-socket system into one you can use with one of those versions as well as accepting some associated penalties, or otherwise simulate a NUMA multi-socket system on a single socket system. How worthwhile that would be would depend on the system and what you are doing I expect.
    – Mokubai ♦
    Commented Dec 5, 2020 at 15:04

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That most likely will affect the guest os' scheduler, because in NUMA multisocket system there're separate caches in each cpu and RAM access times are different, dependent on where the accessed ram is connected to. Any proper OS will account for that (or just cease working, like consumer windows do).

I see no reason to have the reported config to be different from actual allocated hardware other than testing the guest os by tricking it into thinking it's running on some exotic system. Performance-wise it should have no or negative effect.

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