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

Message

Naveen N Rao Feb. 3, 2025, 5:03 p.m. UTC

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