On Mon, 8 Nov 2021 at 20:48, Kele Huang <huangkele@xxxxxxxxxxxxx> wrote: > > Currently, AVIC is disabled if x2apic feature is exposed to guest > or in-kernel PIT is in re-injection mode. > > We can enable AVIC with options: > > Kmod args: > modprobe kvm_amd avic=1 nested=0 npt=1 > QEMU args: > ... -cpu host,-x2apic -global kvm-pit.lost_tick_policy=discard ... > > When LAPIC works in xapic mode, both AVIC and PV_SEND_IPI feature > can accelerate IPI operations for guest. However, the relationship > between AVIC and PV_SEND_IPI feature is not sorted out. > > In logical, AVIC accelerates most of frequently IPI operations > without VMM intervention, while the re-hooking of apic->send_IPI_xxx > from PV_SEND_IPI feature masks out it. People can get confused > if AVIC is enabled while getting lots of hypercall kvm_exits > from IPI. > > In performance, benchmark tool > https://lore.kernel.org/kvm/20171219085010.4081-1-ynorov@xxxxxxxxxxxxxxxxxx/ > shows below results: > > Test env: > CPU: AMD EPYC 7742 64-Core Processor > 2 vCPUs pinned 1:1 > idle=poll > > Test result (average ns per IPI of lots of running): > PV_SEND_IPI : 1860 > AVIC : 1390 > > Besides, disscussions in https://lkml.org/lkml/2021/10/20/423 > do have some solid performance test results to this. > > This patch fixes this by masking out PV_SEND_IPI feature when > AVIC is enabled in setting up of guest vCPUs' CPUID. This is the second time in community you bytedance guys post patches w/o evaluating ipi broadcast performance. https://lore.kernel.org/all/CANRm+Cx597FNRUCyVz1D=B6Vs2GX3Sw57X7Muk+yMpi_hb+v1w@xxxxxxxxxxxxxx/T/#u Wanpeng
