linux-kernel - [RFC] KVM: x86: SVM: don't expose PV_SEND

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Message-Id: <20211108095931.618865-1-huangkele@bytedance.com>
Date:   Mon,  8 Nov 2021 17:59:31 +0800
From:   Kele Huang <huangkele@...edance.com>
To:     pbonzini@...hat.com
Cc:     chaiwen.cc@...edance.com, xieyongji@...edance.com,
        dengliang.1214@...edance.com, pizhenwei@...edance.com,
        wanpengli@...cent.com, seanjc@...gle.com, huangkele@...edance.com,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [RFC] KVM: x86: SVM: don't expose PV_SEND_IPI feature with AVIC

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@...edance.com>
---
 arch/x86/kvm/cpuid.c   |  4 ++--
 arch/x86/kvm/svm/svm.c | 13 +++++++++++++
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 2d70edb0f323..cc22975e2ac5 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -194,8 +194,6 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
 		best->ecx |= XFEATURE_MASK_FPSSE;
 	}
 
-	kvm_update_pv_runtime(vcpu);
-
 	vcpu->arch.maxphyaddr = cpuid_query_maxphyaddr(vcpu);
 	vcpu->arch.reserved_gpa_bits = kvm_vcpu_reserved_gpa_bits_raw(vcpu);
 
@@ -208,6 +206,8 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
 	/* Invoke the vendor callback only after the above state is updated. */
 	static_call(kvm_x86_vcpu_after_set_cpuid)(vcpu);
 
+	kvm_update_pv_runtime(vcpu);
+
 	/*
 	 * Except for the MMU, which needs to do its thing any vendor specific
 	 * adjustments to the reserved GPA bits.
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index b36ca4e476c2..b13bcfb2617c 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -4114,6 +4114,19 @@ static void svm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
 		if (nested && guest_cpuid_has(vcpu, X86_FEATURE_SVM))
 			kvm_request_apicv_update(vcpu->kvm, false,
 						 APICV_INHIBIT_REASON_NESTED);
+
+		if (!guest_cpuid_has(vcpu, X86_FEATURE_X2APIC) &&
+				!(nested && guest_cpuid_has(vcpu, X86_FEATURE_SVM))) {
+			/*
+			 * PV_SEND_IPI feature masks out AVIC acceleration to IPI.
+			 * So, we do not expose PV_SEND_IPI feature to guest when
+			 * AVIC is enabled.
+			 */
+			best = kvm_find_cpuid_entry(vcpu, KVM_CPUID_FEATURES, 0);
+			if (best && enable_apicv &&
+					(best->eax & (1 << KVM_FEATURE_PV_SEND_IPI)))
+				best->eax &= ~(1 << KVM_FEATURE_PV_SEND_IPI);
+		}
 	}
 	init_vmcb_after_set_cpuid(vcpu);
 }
-- 
2.11.0