[PATCH 0/3] KVM: x86: Address performance degradation due to APICv inhibits

Naveen N Rao (AMD)
2025-02-03T17:10:02Z

When AVIC is enabled (kvm_amd avic=1), guests with kernel-irqchip=on and
PIT in reinject mode (default configuration) show degraded performance.
This is because even though APICv is inhibited, we see other inhibits
being set/cleared causing contention on apicv_update_lock. Rework
inhibit code so that apicv_update_lock is not required unless APICv
state is changing. More details in patch 3.

In a test setup with two guests (-smp 16, -netdev tap,vhost=on, -device
virtio-net-pci,mq=on), multiple instances of netperf TCP_RR sending
traffic to the other guest, sar shows:

Without this patch series, kvm_amd avic=0 (default):
Average: IFACE rxpck/s txpck/s rxkB/s txkB/s rxcmp/s txcmp/s rxmcst/s %ifutil
Average: lo 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Average: ens2 1323407.59 1323406.87 250722.47 126654.46 0.00 0.00 0.00 0.00

Without this patch series, kvm_amd avic=1:
Average: IFACE rxpck/s txpck/s rxkB/s txkB/s rxcmp/s txcmp/s rxmcst/s %ifutil
Average: lo 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Average: ens2 1057711.25 1057697.51 200385.83 101225.23 0.00 0.00 0.00 0.00

We see ~20% degradation in packet rate.

With this patch series, kvm_amd avic=1:
Average: IFACE rxpck/s txpck/s rxkB/s txkB/s rxcmp/s txcmp/s rxmcst/s %ifutil
Average: lo 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Average: ens2 1315433.91 1315434.46 249211.99 125891.47 0.00 0.00 0.00 0.00

- Naveen

Naveen N Rao (AMD) (3):
KVM: x86: hyper-v: Convert synic_auto_eoi_used to an atomic
KVM: x86: Remove use of apicv_update_lock when toggling guest debug
state
KVM: x86: Decouple APICv activation state from apicv_inhibit_reasons

arch/x86/include/asm/kvm_host.h | 14 ++--
arch/x86/kvm/hyperv.c | 17 ++---
arch/x86/kvm/x86.c | 125 ++++++++++++++++----------------
3 files changed, 72 insertions(+), 84 deletions(-)

base-commit: eb723766b1030a23c38adf2348b7c3d1409d11f0
--
2.48.1