I've noticed very severe performance degradation on one of the few benchmarks I have been running, and narrowed it down the paravirtualized spinlocks.

Host:
- Fedora 32 with kernel 5.6.5 / Proxmox 6.1-8 (tried both)
- Qemu 4.1 / 4.2 / 5.0.0 rc3 (tried all of these)
- AMD Threadripper 3970X (Zen 2) CPU, 32C/64T
- 64GB of RAM, mostly allocated to 1GB huge pages.

Guest:
- Fedora 32 with kernel 5.6.5

The VM has the following key specs (through libvirt):
  <memory unit="GiB">16</memory>
  <memoryBacking>
    <hugepages/>
  </memoryBacking>
  <vcpu placement="static">48</vcpu>
  <features>
    <acpi/>
    <apic/>
    <vmport state="off"/>
  </features>
  <cpu mode="host-passthrough" check="none">
    <topology sockets="1" dies="1" cores="24" threads="2"/>
    <feature policy="require" name="topoext"/>
  </cpu>

I'm passing through an NVMe drive and running a RocksDB benchmark on it.

Bare metal scores about 1 000 000 Op/s, while the guest scores about 180 000, i.e only 18% of bare metal. CPU usage on the host is about 900%.
CPUID on 0x40000001: eax=0x01003afb

If I add <pvspinlock state="off"/> or <hint-dedicated state="on"/>, then the guest scores very close to bare metal in the benchmark, with CPU usage on the host at 4700%, which means that it's successfully using resources at its disposal.
If I understand correctly, adding the above zeroes the KVM_FEATURE_PV_UNHALT bit in the guest's CPUID, and therefore deactivates the paravirtualized spinlocks.
CPUID on 0x40000001: eax=0x01003a7b

If it's of any help, I also noticed further performance degradation doing the following:
- Pinning the vCPUs:
  <cputune> 
    <vcpupin vcpu="0" cpuset="8"/>
    <vcpupin vcpu="1" cpuset="40"/>
    <vcpupin vcpu="2" cpuset="9"/>
    <vcpupin vcpu="3" cpuset="41"/>
   ...
    <vcpupin vcpu="46" cpuset="31"/>
    <vcpupin vcpu="47" cpuset="63"/>
  </cputune>
- Adding <cache mode="passthrough" />

Enabling AVIC (AMD's equivalent to APICv) on kvm_amd and disabling x2apic and nested virtualization to make it work did not help restore performance.

For comparison, I also tested VMware's ESXi 7.0 and out of the box got a very high percentage of bare metal performance on that benchmark. That led me to think it could be a bug in KVM.

Disclaimer: I'm just a simple enthusiast who doesn't even understand spinlocks, TLB flushes, VM exists, IPI, APIC, etc. But I'm available to run commands and provide you with their outputs.