>Please confirm what's the QEMU command-line being used (especially the -smp and -cpu options), and check if the bug persists if using "-cpu host".
I'm using -cpu host already, here's just the cpu and smp commands: -cpu host,hv_vendor_id=whatever,kvm=off,hv_relaxed,hv_spinlocks=0x1fff,hv_vapic,hv_time,smep=off -smp 12,sockets=1,cores=6,threads=2 The extra commands are just for VGA passthrough, but the problem still occurs with just -cpu host (plus smep=off, problems with booting with it enabled) and the above smp setting. I've attached host output; I'm using a Windows guest and running msinfo32 indicates: AMD Ryzen 1600 Six-Core Processor, 3693 Mhz, 12 Core(s), 12 Logical Processors(s) Suggesting that the guest is seeing the host as 12 cores, 1 thread each, rather than 6 cores, 2 threads each. If Linux guest information would be more helpful, I'll set up a Linux guest as well. ** Attachment added: "Host logs" https://bugs.launchpad.net/qemu/+bug/1703506/+attachment/4917236/+files/qemu-ryzen-smt-logs.tar.gz -- You received this bug notification because you are a member of qemu- devel-ml, which is subscribed to QEMU. https://bugs.launchpad.net/bugs/1703506 Title: SMT not supported by QEMU on AMD Ryzen CPU Status in QEMU: New Bug description: HyperThreading/SMT is supported by AMD Ryzen CPUs but results in this message when setting the topology to threads=2: qemu-system-x86_64: AMD CPU doesn't support hyperthreading. Please configure -smp options properly. Checking in a Windows 10 guest reveals that SMT is not enabled, and from what I understand, QEMU converts the topology from threads to cores internally on AMD CPUs. This appears to cause performance problems in the guest perhaps because programs are assuming that these threads are actual cores. Software: Linux 4.12, qemu 2.9.0 host with KVM enabled, Windows 10 pro guest To manage notifications about this bug go to: https://bugs.launchpad.net/qemu/+bug/1703506/+subscriptions