[PATCH v2 0/3] KVM: Yield to IPI target if necessary

[PATCH v2 0/3] KVM: Yield to IPI target if necessary

[Wanpeng Li]

The idea is from Xen, when sending a call-function IPI-many to vCPUs, 
yield if any of the IPI target vCPUs was preempted. 17% performace 
increase of ebizzy benchmark can be observed in an over-subscribe 
environment. (w/ kvm-pv-tlb disabled, testing TLB flush call-function 
IPI-many since call-function is not easy to be trigged by userspace 
workload).

v1 -> v2:
 * check map is not NULL
 * check map->phys_map[dest_id] is not NULL

Wanpeng Li (3):
  KVM: X86: Implement PV sched yield in linux guest
  KVM: X86: Implement PV sched yield hypercall
  KVM: X86: Expose PV_SCHED_YIELD CPUID feature bit to guest

 Documentation/virtual/kvm/cpuid.txt      |  4 ++++
 Documentation/virtual/kvm/hypercalls.txt | 11 +++++++++++
 arch/x86/include/uapi/asm/kvm_para.h     |  1 +
 arch/x86/kernel/kvm.c                    | 21 +++++++++++++++++++++
 arch/x86/kvm/cpuid.c                     |  3 ++-
 arch/x86/kvm/x86.c                       | 24 ++++++++++++++++++++++++
 include/uapi/linux/kvm_para.h            |  1 +
 7 files changed, 64 insertions(+), 1 deletion(-)

-- 
2.7.4

[PATCH v2 1/3] KVM: X86: Implement PV sched yield in linux guest

[Wanpeng Li]

From: Wanpeng Li <wanpengli@tencent.com>

When sending a call-function IPI-many to vCPUs, yield if any of
the IPI target vCPUs was preempted, we just select the first
preempted target vCPU which we found since the state of target
vCPUs can change underneath and to avoid race conditions.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
---
 Documentation/virtual/kvm/hypercalls.txt | 11 +++++++++++
 arch/x86/include/uapi/asm/kvm_para.h     |  1 +
 arch/x86/kernel/kvm.c                    | 21 +++++++++++++++++++++
 include/uapi/linux/kvm_para.h            |  1 +
 4 files changed, 34 insertions(+)

diff --git a/Documentation/virtual/kvm/hypercalls.txt b/Documentation/virtual/kvm/hypercalls.txt
index da24c13..da21065 100644
--- a/Documentation/virtual/kvm/hypercalls.txt
+++ b/Documentation/virtual/kvm/hypercalls.txt
@@ -141,3 +141,14 @@ a0 corresponds to the APIC ID in the third argument (a2), bit 1
 corresponds to the APIC ID a2+1, and so on.
 
 Returns the number of CPUs to which the IPIs were delivered successfully.
+
+7. KVM_HC_SCHED_YIELD
+------------------------
+Architecture: x86
+Status: active
+Purpose: Hypercall used to yield if the IPI target vCPU is preempted
+
+a0: destination APIC ID
+
+Usage example: When sending a call-function IPI-many to vCPUs, yield if
+any of the IPI target vCPUs was preempted.
diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h
index 19980ec..d0bf77c 100644
--- a/arch/x86/include/uapi/asm/kvm_para.h
+++ b/arch/x86/include/uapi/asm/kvm_para.h
@@ -29,6 +29,7 @@
 #define KVM_FEATURE_PV_TLB_FLUSH	9
 #define KVM_FEATURE_ASYNC_PF_VMEXIT	10
 #define KVM_FEATURE_PV_SEND_IPI	11
+#define KVM_FEATURE_PV_SCHED_YIELD	12
 
 #define KVM_HINTS_REALTIME      0
 
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 3f0cc82..54400c2 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -540,6 +540,21 @@ static void kvm_setup_pv_ipi(void)
 	pr_info("KVM setup pv IPIs\n");
 }
 
+static void kvm_smp_send_call_func_ipi(const struct cpumask *mask)
+{
+	int cpu;
+
+	native_send_call_func_ipi(mask);
+
+	/* Make sure other vCPUs get a chance to run if they need to. */
+	for_each_cpu(cpu, mask) {
+		if (vcpu_is_preempted(cpu)) {
+			kvm_hypercall1(KVM_HC_SCHED_YIELD, per_cpu(x86_cpu_to_apicid, cpu));
+			break;
+		}
+	}
+}
+
 static void __init kvm_smp_prepare_cpus(unsigned int max_cpus)
 {
 	native_smp_prepare_cpus(max_cpus);
@@ -651,6 +666,12 @@ static void __init kvm_guest_init(void)
 #ifdef CONFIG_SMP
 	smp_ops.smp_prepare_cpus = kvm_smp_prepare_cpus;
 	smp_ops.smp_prepare_boot_cpu = kvm_smp_prepare_boot_cpu;
+	if (kvm_para_has_feature(KVM_FEATURE_PV_SCHED_YIELD) &&
+	    !kvm_para_has_hint(KVM_HINTS_REALTIME) &&
+	    kvm_para_has_feature(KVM_FEATURE_STEAL_TIME)) {
+		smp_ops.send_call_func_ipi = kvm_smp_send_call_func_ipi;
+		pr_info("KVM setup pv sched yield\n");
+	}
 	if (cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN, "x86/kvm:online",
 				      kvm_cpu_online, kvm_cpu_down_prepare) < 0)
 		pr_err("kvm_guest: Failed to install cpu hotplug callbacks\n");
diff --git a/include/uapi/linux/kvm_para.h b/include/uapi/linux/kvm_para.h
index 6c0ce49..8b86609 100644
--- a/include/uapi/linux/kvm_para.h
+++ b/include/uapi/linux/kvm_para.h
@@ -28,6 +28,7 @@
 #define KVM_HC_MIPS_CONSOLE_OUTPUT	8
 #define KVM_HC_CLOCK_PAIRING		9
 #define KVM_HC_SEND_IPI		10
+#define KVM_HC_SCHED_YIELD		11
 
 /*
  * hypercalls use architecture specific
-- 
2.7.4

[PATCH v2 2/3] KVM: X86: Implement PV sched yield hypercall

[Wanpeng Li]

From: Wanpeng Li <wanpengli@tencent.com>

The target vCPUs are in runnable state after vcpu_kick and suitable 
as a yield target. This patch implements the sched yield hypercall.

17% performace increase of ebizzy benchmark can be observed in an 
over-subscribe environment. (w/ kvm-pv-tlb disabled, testing TLB flush 
call-function IPI-many since call-function is not easy to be trigged 
by userspace workload).

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
---
 arch/x86/kvm/x86.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index e7e57de..2ceef51 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7172,6 +7172,26 @@ void kvm_vcpu_deactivate_apicv(struct kvm_vcpu *vcpu)
 	kvm_x86_ops->refresh_apicv_exec_ctrl(vcpu);
 }
 
+void kvm_sched_yield(struct kvm *kvm, u64 dest_id)
+{
+	struct kvm_vcpu *target;
+	struct kvm_apic_map *map;
+
+	rcu_read_lock();
+	map = rcu_dereference(kvm->arch.apic_map);
+
+	if (unlikely(!map))
+		goto out;
+
+	if (map->phys_map[dest_id]->vcpu) {
+		target = map->phys_map[dest_id]->vcpu;
+		kvm_vcpu_yield_to(target);
+	}
+
+out:
+	rcu_read_unlock();
+}
+
 int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
 {
 	unsigned long nr, a0, a1, a2, a3, ret;
@@ -7218,6 +7238,10 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
 	case KVM_HC_SEND_IPI:
 		ret = kvm_pv_send_ipi(vcpu->kvm, a0, a1, a2, a3, op_64_bit);
 		break;
+	case KVM_HC_SCHED_YIELD:
+		kvm_sched_yield(vcpu->kvm, a0);
+		ret = 0;
+		break;
 	default:
 		ret = -KVM_ENOSYS;
 		break;
-- 
2.7.4

[PATCH RESEND v2] KVM: X86: Implement PV sched yield hypercall

[Wanpeng Li]

From: Wanpeng Li <wanpengli@tencent.com>

The target vCPUs are in runnable state after vcpu_kick and suitable 
as a yield target. This patch implements the sched yield hypercall.

17% performance increase of ebizzy benchmark can be observed in an 
over-subscribe environment. (w/ kvm-pv-tlb disabled, testing TLB flush 
call-function IPI-many since call-function is not easy to be trigged 
by userspace workload).

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
---
 arch/x86/kvm/x86.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index e7e57de..2f9ec08 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7172,6 +7172,28 @@ void kvm_vcpu_deactivate_apicv(struct kvm_vcpu *vcpu)
 	kvm_x86_ops->refresh_apicv_exec_ctrl(vcpu);
 }
 
+void kvm_sched_yield(struct kvm *kvm, u64 dest_id)
+{
+	struct kvm_vcpu *target;
+	struct kvm_apic_map *map;
+
+	rcu_read_lock();
+	map = rcu_dereference(kvm->arch.apic_map);
+
+	if (unlikely(!map))
+		goto out;
+
+	if (map->phys_map[dest_id]->vcpu) {
+		target = map->phys_map[dest_id]->vcpu;
+		rcu_read_unlock();
+		kvm_vcpu_yield_to(target);
+	}
+
+out:
+	if (!target)
+		rcu_read_unlock();
+}
+
 int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
 {
 	unsigned long nr, a0, a1, a2, a3, ret;
@@ -7218,6 +7240,10 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
 	case KVM_HC_SEND_IPI:
 		ret = kvm_pv_send_ipi(vcpu->kvm, a0, a1, a2, a3, op_64_bit);
 		break;
+	case KVM_HC_SCHED_YIELD:
+		kvm_sched_yield(vcpu->kvm, a0);
+		ret = 0;
+		break;
 	default:
 		ret = -KVM_ENOSYS;
 		break;
-- 
2.7.4

Re: [PATCH RESEND v2] KVM: X86: Implement PV sched yield hypercall

[kbuild test robot]

[-- Attachment #1: Type: text/plain, Size: 3056 bytes --]

Hi Wanpeng,

Thank you for the patch! Yet something to improve:

[auto build test ERROR on kvm/linux-next]
[also build test ERROR on v5.2-rc2 next-20190524]
[if your patch is applied to the wrong git tree, please drop us a note to help improve the system]

url:    https://github.com/0day-ci/linux/commits/Wanpeng-Li/KVM-X86-Implement-PV-sched-yield-hypercall/20190528-132021
base:   https://git.kernel.org/pub/scm/virt/kvm/kvm.git linux-next
config: x86_64-allyesconfig (attached as .config)
compiler: gcc-7 (Debian 7.3.0-1) 7.3.0
reproduce:
        # save the attached .config to linux build tree
        make ARCH=x86_64 

If you fix the issue, kindly add following tag
Reported-by: kbuild test robot <lkp@intel.com>

All errors (new ones prefixed by >>):

   arch/x86//kvm/x86.c: In function 'kvm_emulate_hypercall':
>> arch/x86//kvm/x86.c:7243:7: error: 'KVM_HC_SCHED_YIELD' undeclared (first use in this function); did you mean 'KVM_HC_SEND_IPI'?
     case KVM_HC_SCHED_YIELD:
          ^~~~~~~~~~~~~~~~~~
          KVM_HC_SEND_IPI
   arch/x86//kvm/x86.c:7243:7: note: each undeclared identifier is reported only once for each function it appears in

vim +7243 arch/x86//kvm/x86.c

  7196	
  7197	int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
  7198	{
  7199		unsigned long nr, a0, a1, a2, a3, ret;
  7200		int op_64_bit;
  7201	
  7202		if (kvm_hv_hypercall_enabled(vcpu->kvm))
  7203			return kvm_hv_hypercall(vcpu);
  7204	
  7205		nr = kvm_rax_read(vcpu);
  7206		a0 = kvm_rbx_read(vcpu);
  7207		a1 = kvm_rcx_read(vcpu);
  7208		a2 = kvm_rdx_read(vcpu);
  7209		a3 = kvm_rsi_read(vcpu);
  7210	
  7211		trace_kvm_hypercall(nr, a0, a1, a2, a3);
  7212	
  7213		op_64_bit = is_64_bit_mode(vcpu);
  7214		if (!op_64_bit) {
  7215			nr &= 0xFFFFFFFF;
  7216			a0 &= 0xFFFFFFFF;
  7217			a1 &= 0xFFFFFFFF;
  7218			a2 &= 0xFFFFFFFF;
  7219			a3 &= 0xFFFFFFFF;
  7220		}
  7221	
  7222		if (kvm_x86_ops->get_cpl(vcpu) != 0) {
  7223			ret = -KVM_EPERM;
  7224			goto out;
  7225		}
  7226	
  7227		switch (nr) {
  7228		case KVM_HC_VAPIC_POLL_IRQ:
  7229			ret = 0;
  7230			break;
  7231		case KVM_HC_KICK_CPU:
  7232			kvm_pv_kick_cpu_op(vcpu->kvm, a0, a1);
  7233			ret = 0;
  7234			break;
  7235	#ifdef CONFIG_X86_64
  7236		case KVM_HC_CLOCK_PAIRING:
  7237			ret = kvm_pv_clock_pairing(vcpu, a0, a1);
  7238			break;
  7239	#endif
  7240		case KVM_HC_SEND_IPI:
  7241			ret = kvm_pv_send_ipi(vcpu->kvm, a0, a1, a2, a3, op_64_bit);
  7242			break;
> 7243		case KVM_HC_SCHED_YIELD:
  7244			kvm_sched_yield(vcpu->kvm, a0);
  7245			ret = 0;
  7246			break;
  7247		default:
  7248			ret = -KVM_ENOSYS;
  7249			break;
  7250		}
  7251	out:
  7252		if (!op_64_bit)
  7253			ret = (u32)ret;
  7254		kvm_rax_write(vcpu, ret);
  7255	
  7256		++vcpu->stat.hypercalls;
  7257		return kvm_skip_emulated_instruction(vcpu);
  7258	}
  7259	EXPORT_SYMBOL_GPL(kvm_emulate_hypercall);
  7260	

---
0-DAY kernel test infrastructure                Open Source Technology Center
https://lists.01.org/pipermail/kbuild-all                   Intel Corporation

[-- Attachment #2: .config.gz --]
[-- Type: application/gzip, Size: 68819 bytes --]

Re: [PATCH RESEND v2] KVM: X86: Implement PV sched yield hypercall

[Wanpeng Li]

On Tue, 28 May 2019 at 14:08, kbuild test robot <lkp@intel.com> wrote:
>
> Hi Wanpeng,
>
> Thank you for the patch! Yet something to improve:
>
> [auto build test ERROR on kvm/linux-next]
> [also build test ERROR on v5.2-rc2 next-20190524]
> [if your patch is applied to the wrong git tree, please drop us a note to help improve the system]
>
> url:    https://github.com/0day-ci/linux/commits/Wanpeng-Li/KVM-X86-Implement-PV-sched-yield-hypercall/20190528-132021
> base:   https://git.kernel.org/pub/scm/virt/kvm/kvm.git linux-next
> config: x86_64-allyesconfig (attached as .config)
> compiler: gcc-7 (Debian 7.3.0-1) 7.3.0
> reproduce:
>         # save the attached .config to linux build tree
>         make ARCH=x86_64
>
> If you fix the issue, kindly add following tag
> Reported-by: kbuild test robot <lkp@intel.com>
>
> All errors (new ones prefixed by >>):
>
>    arch/x86//kvm/x86.c: In function 'kvm_emulate_hypercall':
> >> arch/x86//kvm/x86.c:7243:7: error: 'KVM_HC_SCHED_YIELD' undeclared (first use in this function); did you mean 'KVM_HC_SEND_IPI'?
>      case KVM_HC_SCHED_YIELD:
>           ^~~~~~~~~~~~~~~~~~
>           KVM_HC_SEND_IPI
>    arch/x86//kvm/x86.c:7243:7: note: each undeclared identifier is reported only once for each function it appears in

It's a false report, it is declared in patch 1/3.

>
> vim +7243 arch/x86//kvm/x86.c
>
>   7196
>   7197  int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
>   7198  {
>   7199          unsigned long nr, a0, a1, a2, a3, ret;
>   7200          int op_64_bit;
>   7201
>   7202          if (kvm_hv_hypercall_enabled(vcpu->kvm))
>   7203                  return kvm_hv_hypercall(vcpu);
>   7204
>   7205          nr = kvm_rax_read(vcpu);
>   7206          a0 = kvm_rbx_read(vcpu);
>   7207          a1 = kvm_rcx_read(vcpu);
>   7208          a2 = kvm_rdx_read(vcpu);
>   7209          a3 = kvm_rsi_read(vcpu);
>   7210
>   7211          trace_kvm_hypercall(nr, a0, a1, a2, a3);
>   7212
>   7213          op_64_bit = is_64_bit_mode(vcpu);
>   7214          if (!op_64_bit) {
>   7215                  nr &= 0xFFFFFFFF;
>   7216                  a0 &= 0xFFFFFFFF;
>   7217                  a1 &= 0xFFFFFFFF;
>   7218                  a2 &= 0xFFFFFFFF;
>   7219                  a3 &= 0xFFFFFFFF;
>   7220          }
>   7221
>   7222          if (kvm_x86_ops->get_cpl(vcpu) != 0) {
>   7223                  ret = -KVM_EPERM;
>   7224                  goto out;
>   7225          }
>   7226
>   7227          switch (nr) {
>   7228          case KVM_HC_VAPIC_POLL_IRQ:
>   7229                  ret = 0;
>   7230                  break;
>   7231          case KVM_HC_KICK_CPU:
>   7232                  kvm_pv_kick_cpu_op(vcpu->kvm, a0, a1);
>   7233                  ret = 0;
>   7234                  break;
>   7235  #ifdef CONFIG_X86_64
>   7236          case KVM_HC_CLOCK_PAIRING:
>   7237                  ret = kvm_pv_clock_pairing(vcpu, a0, a1);
>   7238                  break;
>   7239  #endif
>   7240          case KVM_HC_SEND_IPI:
>   7241                  ret = kvm_pv_send_ipi(vcpu->kvm, a0, a1, a2, a3, op_64_bit);
>   7242                  break;
> > 7243          case KVM_HC_SCHED_YIELD:
>   7244                  kvm_sched_yield(vcpu->kvm, a0);
>   7245                  ret = 0;
>   7246                  break;
>   7247          default:
>   7248                  ret = -KVM_ENOSYS;
>   7249                  break;
>   7250          }
>   7251  out:
>   7252          if (!op_64_bit)
>   7253                  ret = (u32)ret;
>   7254          kvm_rax_write(vcpu, ret);
>   7255
>   7256          ++vcpu->stat.hypercalls;
>   7257          return kvm_skip_emulated_instruction(vcpu);
>   7258  }
>   7259  EXPORT_SYMBOL_GPL(kvm_emulate_hypercall);
>   7260
>
> ---
> 0-DAY kernel test infrastructure                Open Source Technology Center
> https://lists.01.org/pipermail/kbuild-all                   Intel Corporation

Re: [PATCH RESEND v2] KVM: X86: Implement PV sched yield hypercall

[kbuild test robot]

Hi Wanpeng,

Thank you for the patch! Perhaps something to improve:

[auto build test WARNING on kvm/linux-next]
[also build test WARNING on v5.2-rc2 next-20190524]
[if your patch is applied to the wrong git tree, please drop us a note to help improve the system]

url:    https://github.com/0day-ci/linux/commits/Wanpeng-Li/KVM-X86-Implement-PV-sched-yield-hypercall/20190528-132021
base:   https://git.kernel.org/pub/scm/virt/kvm/kvm.git linux-next
reproduce:
        # apt-get install sparse
        # sparse version: v0.6.1-rc1-7-g2b96cd8-dirty
        make ARCH=x86_64 allmodconfig
        make C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__'

If you fix the issue, kindly add following tag
Reported-by: kbuild test robot <lkp@intel.com>


sparse warnings: (new ones prefixed by >>)

   arch/x86/kvm/x86.c:2379:38: sparse: sparse: incorrect type in argument 1 (different address spaces) @@    expected void const [noderef] <asn:1> * @@    got  const [noderef] <asn:1> * @@
   arch/x86/kvm/x86.c:2379:38: sparse:    expected void const [noderef] <asn:1> *
   arch/x86/kvm/x86.c:2379:38: sparse:    got unsigned char [usertype] *
   arch/x86/kvm/x86.c:7181:15: sparse: sparse: incompatible types in comparison expression (different address spaces):
   arch/x86/kvm/x86.c:7181:15: sparse:    struct kvm_apic_map [noderef] <asn:4> *
   arch/x86/kvm/x86.c:7181:15: sparse:    struct kvm_apic_map *
   arch/x86/kvm/x86.c:7243:14: sparse: sparse: undefined identifier 'KVM_HC_SCHED_YIELD'
>> arch/x86/kvm/x86.c:7243:14: sparse: sparse: incompatible types for 'case' statement
   arch/x86/kvm/x86.c:9408:16: sparse: sparse: incompatible types in comparison expression (different address spaces):
   arch/x86/kvm/x86.c:9408:16: sparse:    struct kvm_apic_map [noderef] <asn:4> *
   arch/x86/kvm/x86.c:9408:16: sparse:    struct kvm_apic_map *
>> arch/x86/kvm/x86.c:7193:9: sparse: sparse: context imbalance in 'kvm_sched_yield' - wrong count at exit
   arch/x86/kvm/x86.c:7243:14: sparse: sparse: Expected constant expression in case statement

vim +/case +7243 arch/x86/kvm/x86.c

  7174	
  7175	void kvm_sched_yield(struct kvm *kvm, u64 dest_id)
  7176	{
  7177		struct kvm_vcpu *target;
  7178		struct kvm_apic_map *map;
  7179	
  7180		rcu_read_lock();
  7181		map = rcu_dereference(kvm->arch.apic_map);
  7182	
  7183		if (unlikely(!map))
  7184			goto out;
  7185	
  7186		if (map->phys_map[dest_id]->vcpu) {
  7187			target = map->phys_map[dest_id]->vcpu;
  7188			rcu_read_unlock();
  7189			kvm_vcpu_yield_to(target);
  7190		}
  7191	
  7192	out:
> 7193		if (!target)
  7194			rcu_read_unlock();
  7195	}
  7196	
  7197	int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
  7198	{
  7199		unsigned long nr, a0, a1, a2, a3, ret;
  7200		int op_64_bit;
  7201	
  7202		if (kvm_hv_hypercall_enabled(vcpu->kvm))
  7203			return kvm_hv_hypercall(vcpu);
  7204	
  7205		nr = kvm_rax_read(vcpu);
  7206		a0 = kvm_rbx_read(vcpu);
  7207		a1 = kvm_rcx_read(vcpu);
  7208		a2 = kvm_rdx_read(vcpu);
  7209		a3 = kvm_rsi_read(vcpu);
  7210	
  7211		trace_kvm_hypercall(nr, a0, a1, a2, a3);
  7212	
  7213		op_64_bit = is_64_bit_mode(vcpu);
  7214		if (!op_64_bit) {
  7215			nr &= 0xFFFFFFFF;
  7216			a0 &= 0xFFFFFFFF;
  7217			a1 &= 0xFFFFFFFF;
  7218			a2 &= 0xFFFFFFFF;
  7219			a3 &= 0xFFFFFFFF;
  7220		}
  7221	
  7222		if (kvm_x86_ops->get_cpl(vcpu) != 0) {
  7223			ret = -KVM_EPERM;
  7224			goto out;
  7225		}
  7226	
  7227		switch (nr) {
  7228		case KVM_HC_VAPIC_POLL_IRQ:
  7229			ret = 0;
  7230			break;
  7231		case KVM_HC_KICK_CPU:
  7232			kvm_pv_kick_cpu_op(vcpu->kvm, a0, a1);
  7233			ret = 0;
  7234			break;
  7235	#ifdef CONFIG_X86_64
  7236		case KVM_HC_CLOCK_PAIRING:
  7237			ret = kvm_pv_clock_pairing(vcpu, a0, a1);
  7238			break;
  7239	#endif
  7240		case KVM_HC_SEND_IPI:
  7241			ret = kvm_pv_send_ipi(vcpu->kvm, a0, a1, a2, a3, op_64_bit);
  7242			break;
> 7243		case KVM_HC_SCHED_YIELD:
  7244			kvm_sched_yield(vcpu->kvm, a0);
  7245			ret = 0;
  7246			break;
  7247		default:
  7248			ret = -KVM_ENOSYS;
  7249			break;
  7250		}
  7251	out:
  7252		if (!op_64_bit)
  7253			ret = (u32)ret;
  7254		kvm_rax_write(vcpu, ret);
  7255	
  7256		++vcpu->stat.hypercalls;
  7257		return kvm_skip_emulated_instruction(vcpu);
  7258	}
  7259	EXPORT_SYMBOL_GPL(kvm_emulate_hypercall);
  7260	

---
0-DAY kernel test infrastructure                Open Source Technology Center
https://lists.01.org/pipermail/kbuild-all                   Intel Corporation

[PATCH RESEND v2 0/3] KVM: Yield to IPI target if necessary

[Wanpeng Li]

The idea is from Xen, when sending a call-function IPI-many to vCPUs, 
yield if any of the IPI target vCPUs was preempted. 17% performance 
increase of ebizzy benchmark can be observed in an over-subscribe 
environment. (w/ kvm-pv-tlb disabled, testing TLB flush call-function 
IPI-many since call-function is not easy to be trigged by userspace 
workload).

v1 -> v2:
 * check map is not NULL
 * check map->phys_map[dest_id] is not NULL
 * make kvm_sched_yield static
 * change dest_id to unsinged long

Wanpeng Li (3):
  KVM: X86: Implement PV sched yield in linux guest
  KVM: X86: Implement PV sched yield hypercall
  KVM: X86: Expose PV_SCHED_YIELD CPUID feature bit to guest

 Documentation/virtual/kvm/cpuid.txt      |  4 ++++
 Documentation/virtual/kvm/hypercalls.txt | 11 +++++++++++
 arch/x86/include/uapi/asm/kvm_para.h     |  1 +
 arch/x86/kernel/kvm.c                    | 21 +++++++++++++++++++++
 arch/x86/kvm/cpuid.c                     |  3 ++-
 arch/x86/kvm/x86.c                       | 26 ++++++++++++++++++++++++++
 include/uapi/linux/kvm_para.h            |  1 +
 7 files changed, 66 insertions(+), 1 deletion(-)

-- 
2.7.4

[PATCH RESEND v2 1/3] KVM: X86: Implement PV sched yield in linux guest

[Wanpeng Li]

From: Wanpeng Li <wanpengli@tencent.com>

When sending a call-function IPI-many to vCPUs, yield if any of
the IPI target vCPUs was preempted, we just select the first
preempted target vCPU which we found since the state of target
vCPUs can change underneath and to avoid race conditions.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
---
 Documentation/virtual/kvm/hypercalls.txt | 11 +++++++++++
 arch/x86/include/uapi/asm/kvm_para.h     |  1 +
 arch/x86/kernel/kvm.c                    | 21 +++++++++++++++++++++
 include/uapi/linux/kvm_para.h            |  1 +
 4 files changed, 34 insertions(+)

diff --git a/Documentation/virtual/kvm/hypercalls.txt b/Documentation/virtual/kvm/hypercalls.txt
index da24c13..da21065 100644
--- a/Documentation/virtual/kvm/hypercalls.txt
+++ b/Documentation/virtual/kvm/hypercalls.txt
@@ -141,3 +141,14 @@ a0 corresponds to the APIC ID in the third argument (a2), bit 1
 corresponds to the APIC ID a2+1, and so on.
 
 Returns the number of CPUs to which the IPIs were delivered successfully.
+
+7. KVM_HC_SCHED_YIELD
+------------------------
+Architecture: x86
+Status: active
+Purpose: Hypercall used to yield if the IPI target vCPU is preempted
+
+a0: destination APIC ID
+
+Usage example: When sending a call-function IPI-many to vCPUs, yield if
+any of the IPI target vCPUs was preempted.
diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h
index 19980ec..d0bf77c 100644
--- a/arch/x86/include/uapi/asm/kvm_para.h
+++ b/arch/x86/include/uapi/asm/kvm_para.h
@@ -29,6 +29,7 @@
 #define KVM_FEATURE_PV_TLB_FLUSH	9
 #define KVM_FEATURE_ASYNC_PF_VMEXIT	10
 #define KVM_FEATURE_PV_SEND_IPI	11
+#define KVM_FEATURE_PV_SCHED_YIELD	12
 
 #define KVM_HINTS_REALTIME      0
 
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 3f0cc82..54400c2 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -540,6 +540,21 @@ static void kvm_setup_pv_ipi(void)
 	pr_info("KVM setup pv IPIs\n");
 }
 
+static void kvm_smp_send_call_func_ipi(const struct cpumask *mask)
+{
+	int cpu;
+
+	native_send_call_func_ipi(mask);
+
+	/* Make sure other vCPUs get a chance to run if they need to. */
+	for_each_cpu(cpu, mask) {
+		if (vcpu_is_preempted(cpu)) {
+			kvm_hypercall1(KVM_HC_SCHED_YIELD, per_cpu(x86_cpu_to_apicid, cpu));
+			break;
+		}
+	}
+}
+
 static void __init kvm_smp_prepare_cpus(unsigned int max_cpus)
 {
 	native_smp_prepare_cpus(max_cpus);
@@ -651,6 +666,12 @@ static void __init kvm_guest_init(void)
 #ifdef CONFIG_SMP
 	smp_ops.smp_prepare_cpus = kvm_smp_prepare_cpus;
 	smp_ops.smp_prepare_boot_cpu = kvm_smp_prepare_boot_cpu;
+	if (kvm_para_has_feature(KVM_FEATURE_PV_SCHED_YIELD) &&
+	    !kvm_para_has_hint(KVM_HINTS_REALTIME) &&
+	    kvm_para_has_feature(KVM_FEATURE_STEAL_TIME)) {
+		smp_ops.send_call_func_ipi = kvm_smp_send_call_func_ipi;
+		pr_info("KVM setup pv sched yield\n");
+	}
 	if (cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN, "x86/kvm:online",
 				      kvm_cpu_online, kvm_cpu_down_prepare) < 0)
 		pr_err("kvm_guest: Failed to install cpu hotplug callbacks\n");
diff --git a/include/uapi/linux/kvm_para.h b/include/uapi/linux/kvm_para.h
index 6c0ce49..8b86609 100644
--- a/include/uapi/linux/kvm_para.h
+++ b/include/uapi/linux/kvm_para.h
@@ -28,6 +28,7 @@
 #define KVM_HC_MIPS_CONSOLE_OUTPUT	8
 #define KVM_HC_CLOCK_PAIRING		9
 #define KVM_HC_SEND_IPI		10
+#define KVM_HC_SCHED_YIELD		11
 
 /*
  * hypercalls use architecture specific
-- 
2.7.4

[PATCH RESEND v2 2/3] KVM: X86: Implement PV sched yield hypercall

[Wanpeng Li]

From: Wanpeng Li <wanpengli@tencent.com>

The target vCPUs are in runnable state after vcpu_kick and suitable 
as a yield target. This patch implements the sched yield hypercall.

17% performance increase of ebizzy benchmark can be observed in an 
over-subscribe environment. (w/ kvm-pv-tlb disabled, testing TLB flush 
call-function IPI-many since call-function is not easy to be trigged 
by userspace workload).

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
---
 arch/x86/kvm/x86.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index e7e57de..f73af3b 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7172,6 +7172,28 @@ void kvm_vcpu_deactivate_apicv(struct kvm_vcpu *vcpu)
 	kvm_x86_ops->refresh_apicv_exec_ctrl(vcpu);
 }
 
+static void kvm_sched_yield(struct kvm *kvm, unsigned long dest_id)
+{
+	struct kvm_vcpu *target;
+	struct kvm_apic_map *map;
+
+	rcu_read_lock();
+	map = rcu_dereference(kvm->arch.apic_map);
+
+	if (unlikely(!map))
+		goto out;
+
+	if (map->phys_map[dest_id]->vcpu) {
+		target = map->phys_map[dest_id]->vcpu;
+		rcu_read_unlock();
+		kvm_vcpu_yield_to(target);
+	}
+
+out:
+	if (!target)
+		rcu_read_unlock();
+}
+
 int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
 {
 	unsigned long nr, a0, a1, a2, a3, ret;
@@ -7218,6 +7240,10 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
 	case KVM_HC_SEND_IPI:
 		ret = kvm_pv_send_ipi(vcpu->kvm, a0, a1, a2, a3, op_64_bit);
 		break;
+	case KVM_HC_SCHED_YIELD:
+		kvm_sched_yield(vcpu->kvm, a0);
+		ret = 0;
+		break;
 	default:
 		ret = -KVM_ENOSYS;
 		break;
-- 
2.7.4

[PATCH RESEND v2 3/3] KVM: X86: Expose PV_SCHED_YIELD CPUID feature bit to guest

[Wanpeng Li]

From: Wanpeng Li <wanpengli@tencent.com>

Expose PV_SCHED_YIELD feature bit to guest, the guest can check this
feature bit before using paravirtualized sched yield.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
---
 Documentation/virtual/kvm/cpuid.txt | 4 ++++
 arch/x86/kvm/cpuid.c                | 3 ++-
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/Documentation/virtual/kvm/cpuid.txt b/Documentation/virtual/kvm/cpuid.txt
index 97ca194..1c39683 100644
--- a/Documentation/virtual/kvm/cpuid.txt
+++ b/Documentation/virtual/kvm/cpuid.txt
@@ -66,6 +66,10 @@ KVM_FEATURE_PV_SEND_IPI            ||    11 || guest checks this feature bit
                                    ||       || before using paravirtualized
                                    ||       || send IPIs.
 ------------------------------------------------------------------------------
+KVM_FEATURE_PV_SHED_YIELD          ||    12 || guest checks this feature bit
+                                   ||       || before using paravirtualized
+                                   ||       || sched yield.
+------------------------------------------------------------------------------
 KVM_FEATURE_CLOCKSOURCE_STABLE_BIT ||    24 || host will warn if no guest-side
                                    ||       || per-cpu warps are expected in
                                    ||       || kvmclock.
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index e18a9f9..c018fc8 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -643,7 +643,8 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
 			     (1 << KVM_FEATURE_PV_UNHALT) |
 			     (1 << KVM_FEATURE_PV_TLB_FLUSH) |
 			     (1 << KVM_FEATURE_ASYNC_PF_VMEXIT) |
-			     (1 << KVM_FEATURE_PV_SEND_IPI);
+			     (1 << KVM_FEATURE_PV_SEND_IPI) |
+			     (1 << KVM_FEATURE_PV_SCHED_YIELD);
 
 		if (sched_info_on())
 			entry->eax |= (1 << KVM_FEATURE_STEAL_TIME);
-- 
2.7.4

Re: [PATCH v2 2/3] KVM: X86: Implement PV sched yield hypercall

[Christian Borntraeger]

On 28.05.19 02:53, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@tencent.com>
> 
> The target vCPUs are in runnable state after vcpu_kick and suitable 
> as a yield target. This patch implements the sched yield hypercall.
> 
> 17% performace increase of ebizzy benchmark can be observed in an 
> over-subscribe environment. (w/ kvm-pv-tlb disabled, testing TLB flush 
> call-function IPI-many since call-function is not easy to be trigged 
> by userspace workload).
> 
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Radim Krčmář <rkrcmar@redhat.com>
> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>

FWIW, we do have a similar interface in s390.

See arch/s390/kvm/diag.c  __diag_time_slice_end_directed for our implementation.
> ---
>  arch/x86/kvm/x86.c | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index e7e57de..2ceef51 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -7172,6 +7172,26 @@ void kvm_vcpu_deactivate_apicv(struct kvm_vcpu *vcpu)
>  	kvm_x86_ops->refresh_apicv_exec_ctrl(vcpu);
>  }
> 
> +void kvm_sched_yield(struct kvm *kvm, u64 dest_id)
> +{
> +	struct kvm_vcpu *target;
> +	struct kvm_apic_map *map;
> +
> +	rcu_read_lock();
> +	map = rcu_dereference(kvm->arch.apic_map);
> +
> +	if (unlikely(!map))
> +		goto out;
> +
> +	if (map->phys_map[dest_id]->vcpu) {
> +		target = map->phys_map[dest_id]->vcpu;
> +		kvm_vcpu_yield_to(target);
> +	}
> +
> +out:
> +	rcu_read_unlock();
> +}
> +
>  int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
>  {
>  	unsigned long nr, a0, a1, a2, a3, ret;
> @@ -7218,6 +7238,10 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
>  	case KVM_HC_SEND_IPI:
>  		ret = kvm_pv_send_ipi(vcpu->kvm, a0, a1, a2, a3, op_64_bit);
>  		break;
> +	case KVM_HC_SCHED_YIELD:
> +		kvm_sched_yield(vcpu->kvm, a0);
> +		ret = 0;
> +		break;
>  	default:
>  		ret = -KVM_ENOSYS;
>  		break;
> 

Re: [PATCH v2 2/3] KVM: X86: Implement PV sched yield hypercall

[Wanpeng Li]

On Tue, 28 May 2019 at 17:12, Christian Borntraeger
<borntraeger@de.ibm.com> wrote:
>
> On 28.05.19 02:53, Wanpeng Li wrote:
> > From: Wanpeng Li <wanpengli@tencent.com>
> >
> > The target vCPUs are in runnable state after vcpu_kick and suitable
> > as a yield target. This patch implements the sched yield hypercall.
> >
> > 17% performace increase of ebizzy benchmark can be observed in an
> > over-subscribe environment. (w/ kvm-pv-tlb disabled, testing TLB flush
> > call-function IPI-many since call-function is not easy to be trigged
> > by userspace workload).
> >
> > Cc: Paolo Bonzini <pbonzini@redhat.com>
> > Cc: Radim Krčmář <rkrcmar@redhat.com>
> > Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
>
> FWIW, we do have a similar interface in s390.
>
> See arch/s390/kvm/diag.c  __diag_time_slice_end_directed for our implementation.

Good to know this. :)

Regards,
Wanpeng Li

Re: [PATCH v2 2/3] KVM: X86: Implement PV sched yield hypercall

[Liran Alon]

> On 28 May 2019, at 3:53, Wanpeng Li <kernellwp@gmail.com> wrote:
> 
> From: Wanpeng Li <wanpengli@tencent.com>
> 
> The target vCPUs are in runnable state after vcpu_kick and suitable 
> as a yield target. This patch implements the sched yield hypercall.
> 
> 17% performace increase of ebizzy benchmark can be observed in an 
> over-subscribe environment. (w/ kvm-pv-tlb disabled, testing TLB flush 
> call-function IPI-many since call-function is not easy to be trigged 
> by userspace workload).
> 
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Radim Krčmář <rkrcmar@redhat.com>
> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> ---
> arch/x86/kvm/x86.c | 24 ++++++++++++++++++++++++
> 1 file changed, 24 insertions(+)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index e7e57de..2ceef51 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -7172,6 +7172,26 @@ void kvm_vcpu_deactivate_apicv(struct kvm_vcpu *vcpu)
> 	kvm_x86_ops->refresh_apicv_exec_ctrl(vcpu);
> }
> 
> +void kvm_sched_yield(struct kvm *kvm, u64 dest_id)
> +{
> +	struct kvm_vcpu *target;
> +	struct kvm_apic_map *map;
> +
> +	rcu_read_lock();
> +	map = rcu_dereference(kvm->arch.apic_map);
> +
> +	if (unlikely(!map))
> +		goto out;
> +

We should have a bounds-check here on “dest_id”.

-Liran

> +	if (map->phys_map[dest_id]->vcpu) {
> +		target = map->phys_map[dest_id]->vcpu;
> +		kvm_vcpu_yield_to(target);
> +	}
> +
> +out:
> +	rcu_read_unlock();
> +}
> +
> int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
> {
> 	unsigned long nr, a0, a1, a2, a3, ret;
> @@ -7218,6 +7238,10 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
> 	case KVM_HC_SEND_IPI:
> 		ret = kvm_pv_send_ipi(vcpu->kvm, a0, a1, a2, a3, op_64_bit);
> 		break;
> +	case KVM_HC_SCHED_YIELD:
> +		kvm_sched_yield(vcpu->kvm, a0);
> +		ret = 0;
> +		break;
> 	default:
> 		ret = -KVM_ENOSYS;
> 		break;
> -- 
> 2.7.4
> 

Re: [PATCH v2 2/3] KVM: X86: Implement PV sched yield hypercall

[Wanpeng Li]

On Wed, 29 May 2019 at 20:28, Liran Alon <liran.alon@oracle.com> wrote:
>
>
>
> > On 28 May 2019, at 3:53, Wanpeng Li <kernellwp@gmail.com> wrote:
> >
> > From: Wanpeng Li <wanpengli@tencent.com>
> >
> > The target vCPUs are in runnable state after vcpu_kick and suitable
> > as a yield target. This patch implements the sched yield hypercall.
> >
> > 17% performace increase of ebizzy benchmark can be observed in an
> > over-subscribe environment. (w/ kvm-pv-tlb disabled, testing TLB flush
> > call-function IPI-many since call-function is not easy to be trigged
> > by userspace workload).
> >
> > Cc: Paolo Bonzini <pbonzini@redhat.com>
> > Cc: Radim Krčmář <rkrcmar@redhat.com>
> > Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> > ---
> > arch/x86/kvm/x86.c | 24 ++++++++++++++++++++++++
> > 1 file changed, 24 insertions(+)
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index e7e57de..2ceef51 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -7172,6 +7172,26 @@ void kvm_vcpu_deactivate_apicv(struct kvm_vcpu *vcpu)
> >       kvm_x86_ops->refresh_apicv_exec_ctrl(vcpu);
> > }
> >
> > +void kvm_sched_yield(struct kvm *kvm, u64 dest_id)
> > +{
> > +     struct kvm_vcpu *target;
> > +     struct kvm_apic_map *map;
> > +
> > +     rcu_read_lock();
> > +     map = rcu_dereference(kvm->arch.apic_map);
> > +
> > +     if (unlikely(!map))
> > +             goto out;
> > +
>
> We should have a bounds-check here on “dest_id”.

Yeah, fix it in v3.

Regards,
Wanpeng Li

[PATCH v2 3/3] KVM: X86: Expose PV_SCHED_YIELD CPUID feature bit to guest

[Wanpeng Li]

From: Wanpeng Li <wanpengli@tencent.com>

Expose PV_SCHED_YIELD feature bit to guest, the guest can check this
feature bit before using paravirtualized sched yield.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
---
 Documentation/virtual/kvm/cpuid.txt | 4 ++++
 arch/x86/kvm/cpuid.c                | 3 ++-
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/Documentation/virtual/kvm/cpuid.txt b/Documentation/virtual/kvm/cpuid.txt
index 97ca194..1c39683 100644
--- a/Documentation/virtual/kvm/cpuid.txt
+++ b/Documentation/virtual/kvm/cpuid.txt
@@ -66,6 +66,10 @@ KVM_FEATURE_PV_SEND_IPI            ||    11 || guest checks this feature bit
                                    ||       || before using paravirtualized
                                    ||       || send IPIs.
 ------------------------------------------------------------------------------
+KVM_FEATURE_PV_SHED_YIELD          ||    12 || guest checks this feature bit
+                                   ||       || before using paravirtualized
+                                   ||       || sched yield.
+------------------------------------------------------------------------------
 KVM_FEATURE_CLOCKSOURCE_STABLE_BIT ||    24 || host will warn if no guest-side
                                    ||       || per-cpu warps are expected in
                                    ||       || kvmclock.
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index e18a9f9..c018fc8 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -643,7 +643,8 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
 			     (1 << KVM_FEATURE_PV_UNHALT) |
 			     (1 << KVM_FEATURE_PV_TLB_FLUSH) |
 			     (1 << KVM_FEATURE_ASYNC_PF_VMEXIT) |
-			     (1 << KVM_FEATURE_PV_SEND_IPI);
+			     (1 << KVM_FEATURE_PV_SEND_IPI) |
+			     (1 << KVM_FEATURE_PV_SCHED_YIELD);
 
 		if (sched_info_on())
 			entry->eax |= (1 << KVM_FEATURE_STEAL_TIME);
-- 
2.7.4