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. Signed-off-by: Jiaqing Zhao <jiaqing.zhao@xxxxxxxxx> --- arch/x86/kernel/apic/apic_flat_64.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) diff --git a/arch/x86/kernel/apic/apic_flat_64.c b/arch/x86/kernel/apic/apic_flat_64.c index 8f72b4351c9f..3196bf220230 100644 --- a/arch/x86/kernel/apic/apic_flat_64.c +++ b/arch/x86/kernel/apic/apic_flat_64.c @@ -64,6 +64,13 @@ static void flat_send_IPI_mask(const struct cpumask *cpumask, int vector) _flat_send_IPI_mask(mask, vector); } +static void flat_send_IPI_single(int cpu, int vector) +{ + unsigned long mask = cpumask_bits(cpumask_of(cpu))[0]; + + _flat_send_IPI_mask(mask, vector); +} + static void flat_send_IPI_mask_allbutself(const struct cpumask *cpumask, int vector) { @@ -132,7 +139,7 @@ static struct apic apic_flat __ro_after_init = { .calc_dest_apicid = apic_flat_calc_apicid, - .send_IPI = default_send_IPI_single, + .send_IPI = flat_send_IPI_single, .send_IPI_mask = flat_send_IPI_mask, .send_IPI_mask_allbutself = flat_send_IPI_mask_allbutself, .send_IPI_allbutself = default_send_IPI_allbutself, -- 2.27.0
