LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
From: Chao Gao <chao.gao@intel.com>
To: zhenwei pi <pizhenwei@bytedance.com>
Cc: Wanpeng Li <kernellwp@gmail.com>,
	Maxim Levitsky <mlevitsk@redhat.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Kele Huang <huangkele@bytedance.com>,
	chaiwen.cc@bytedance.com, xieyongji@bytedance.com,
	dengliang.1214@bytedance.com, Wanpeng Li <wanpengli@tencent.com>,
	Sean Christopherson <seanjc@google.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	the arch/x86 maintainers <x86@kernel.org>,
	"H. Peter Anvin" <hpa@zytor.com>, kvm <kvm@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: Re: [RFC] KVM: x86: SVM: don't expose PV_SEND_IPI feature with AVIC
Date: Tue, 16 Nov 2021 17:06:05 +0800	[thread overview]
Message-ID: <20211116090604.GA12758@gao-cwp> (raw)
In-Reply-To: <d65fbd73-7612-8348-2fd8-8da0f5e2a3c0@bytedance.com>

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@redhat.com> 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@caviumnetworks.com/
>> > > > 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@bytedance.com>
>> > > 
>> > > 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@intel.com>
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@intel.com>
---
 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



  reply	other threads:[~2021-11-16  8:56 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-11-08  9:59 Kele Huang
2021-11-08 10:30 ` Paolo Bonzini
2021-11-08 11:08   ` Maxim Levitsky
2021-11-08 11:14     ` zhenwei pi
2021-11-08 11:18       ` Paolo Bonzini
2021-11-16  2:48     ` Wanpeng Li
2021-11-16  2:56       ` zhenwei pi
2021-11-16  9:06         ` Chao Gao [this message]
2021-11-16  9:30           ` [External] " 黄科乐
2021-11-16  9:30           ` Wanpeng Li
     [not found]           ` <CAKUug92xp7mU_KB66jGtdYRhgQpgfCm67r+3kMOMdbrGOrTQcA@mail.gmail.com>
2021-11-16 15:57             ` [External] " Sean Christopherson
2021-11-08 10:45 ` Vitaly Kuznetsov
2021-11-16  2:04 ` Wanpeng Li

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20211116090604.GA12758@gao-cwp \
    --to=chao.gao@intel.com \
    --cc=bp@alien8.de \
    --cc=chaiwen.cc@bytedance.com \
    --cc=dave.hansen@linux.intel.com \
    --cc=dengliang.1214@bytedance.com \
    --cc=hpa@zytor.com \
    --cc=huangkele@bytedance.com \
    --cc=jmattson@google.com \
    --cc=joro@8bytes.org \
    --cc=kernellwp@gmail.com \
    --cc=kvm@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mingo@redhat.com \
    --cc=mlevitsk@redhat.com \
    --cc=pbonzini@redhat.com \
    --cc=pizhenwei@bytedance.com \
    --cc=seanjc@google.com \
    --cc=tglx@linutronix.de \
    --cc=vkuznets@redhat.com \
    --cc=wanpengli@tencent.com \
    --cc=x86@kernel.org \
    --cc=xieyongji@bytedance.com \
    --subject='Re: Re: [RFC] KVM: x86: SVM: don'\''t expose PV_SEND_IPI feature with AVIC' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).