linux-kernel - [PATCH 1/2] KVM: X86: Add dedicated pCPU hint PV

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Date:   Thu,  1 Feb 2018 23:11:12 -0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: [PATCH 1/2] KVM: X86: Add dedicated pCPU hint PV_DEDICATED

From: Wanpeng Li <wanpengli@...cent.com>

Waiman Long mentioned that:

 Generally speaking, unfair lock performs well for VMs with a small
 number of vCPUs. Native qspinlock may perform better than pvqspinlock
 if there is vCPU pinning and there is no vCPU over-commitment.

This patch adds a performance hint to allow hypervisor admin to choose
the qspinlock to be used when a dedicated pCPU is available.

PV_DEDICATED = 1, PV_UNHALT = anything: default is qspinlock
PV_DEDICATED = 0, PV_UNHALT = 1: default is Hybrid PV queued/unfair lock
PV_DEDICATED = 0, PV_UNHALT = 0: default is tas

Cc: Paolo Bonzini <pbonzini@...hat.com>
Cc: Radim Krčmář <rkrcmar@...hat.com>
Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
---
 Documentation/virtual/kvm/cpuid.txt  | 6 ++++++
 arch/x86/include/uapi/asm/kvm_para.h | 1 +
 arch/x86/kernel/kvm.c                | 6 ++++++
 3 files changed, 13 insertions(+)

diff --git a/Documentation/virtual/kvm/cpuid.txt b/Documentation/virtual/kvm/cpuid.txt
index 87a7506..c0740b1 100644
--- a/Documentation/virtual/kvm/cpuid.txt
+++ b/Documentation/virtual/kvm/cpuid.txt
@@ -54,6 +54,12 @@ KVM_FEATURE_PV_UNHALT              ||     7 || guest checks this feature bit
                                    ||       || before enabling paravirtualized
                                    ||       || spinlock support.
 ------------------------------------------------------------------------------
+KVM_FEATURE_PV_DEDICATED           ||     8 || guest checks this feature bit
+                                   ||       || to determine if they run on
+                                   ||       || dedicated vCPUs, allowing opti-
+                                   ||       || mizations such as usage of
+                                   ||       || qspinlocks.
+------------------------------------------------------------------------------
 KVM_FEATURE_PV_TLB_FLUSH           ||     9 || guest checks this feature bit
                                    ||       || before enabling paravirtualized
                                    ||       || tlb flush.
diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h
index 6cfa9c8..9a5ef67 100644
--- a/arch/x86/include/uapi/asm/kvm_para.h
+++ b/arch/x86/include/uapi/asm/kvm_para.h
@@ -25,6 +25,7 @@
 #define KVM_FEATURE_STEAL_TIME		5
 #define KVM_FEATURE_PV_EOI		6
 #define KVM_FEATURE_PV_UNHALT		7
+#define KVM_FEATURE_PV_DEDICATED	8
 #define KVM_FEATURE_PV_TLB_FLUSH	9
 #define KVM_FEATURE_ASYNC_PF_VMEXIT	10
 
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index aa2b706..6f0e43f 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -726,6 +726,12 @@ void __init kvm_spinlock_init(void)
 {
 	if (!kvm_para_available())
 		return;
+
+	if (kvm_para_has_feature(KVM_FEATURE_PV_DEDICATED)) {
+		static_branch_disable(&virt_spin_lock_key);
+		return;
+	}
+
 	/* Does host kernel support KVM_FEATURE_PV_UNHALT? */
 	if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT))
 		return;
-- 
2.7.4