On Tue, 16 Nov 2021 at 16:56, Chao Gao <chao.gao@xxxxxxxxx> wrote: > > On Tue, Nov 16, 2021 at 10:56:25AM +0800, zhenwei pi wrote: > > > > > >On 11/16/21 10:48 AM, Wanpeng Li wrote: > >> On Mon, 8 Nov 2021 at 22:09, Maxim Levitsky <mlevitsk@xxxxxxxxxx> wrote: > >> > > >> > On Mon, 2021-11-08 at 11:30 +0100, Paolo Bonzini wrote: > >> > > On 11/8/21 10:59, Kele Huang 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. > >> > > > > >> > > > Signed-off-by: Kele Huang <huangkele@xxxxxxxxxxxxx> > >> > > > >> > > AVIC can change across migration. I think we should instead use a new > >> > > KVM_HINTS_* bit (KVM_HINTS_ACCELERATED_LAPIC or something like that). > >> > > The KVM_HINTS_* bits are intended to be changeable across migration, > >> > > even though we don't have for now anything equivalent to the Hyper-V > >> > > reenlightenment interrupt. > >> > > >> > Note that the same issue exists with HyperV. It also has PV APIC, > >> > which is harmful when AVIC is enabled (that is guest uses it instead > >> > of using AVIC, negating AVIC benefits). > >> > > >> > Also note that Intel recently posted IPI virtualizaion, which > >> > will make this issue relevant to APICv too soon. > >> > >> The recently posted Intel IPI virtualization will accelerate unicast > >> ipi but not broadcast ipis, AMD AVIC accelerates unicast ipi well but > >> accelerates broadcast ipis worse than pv ipis. Could we just handle > >> unicast ipi here? > >> > >> Wanpeng > >> > >Depend on the number of target vCPUs, broadcast IPIs gets unstable > >performance on AVIC, and usually worse than PV Send IPI. > >So agree with Wanpeng's point, is it possible to separate single IPI and > >broadcast IPI on a hardware acceleration platform? > > Actually, this is how kernel works in x2apic mode: use PV interface > (hypercall) to send multi-cast IPIs and write ICR MSR directly to send > unicast IPIs. > > But if guest works in xapic mode, both unicast and multi-cast are issued > via PV interface. It is a side-effect introduced by commit aaffcfd1e82d. > > how about just correcting the logic for xapic: > > From 13447b221252b64cd85ed1329f7d917afa54efc8 Mon Sep 17 00:00:00 2001 > From: Jiaqing Zhao <jiaqing.zhao@xxxxxxxxx> > Date: Fri, 9 Apr 2021 13:53:39 +0800 > Subject: [PATCH 1/2] x86/apic/flat: Add specific send IPI logic > > Currently, apic_flat.send_IPI() uses default_send_IPI_single(), which > is a wrapper of apic->send_IPI_mask(). Since commit aaffcfd1e82d > ("KVM: X86: Implement PV IPIs in linux guest"), KVM PV IPI driver will > override apic->send_IPI_mask(), and may cause unwated side effects. > > This patch removes such side effects by creating a specific send_IPI > method. This looks reasonable to me. Wanpeng
