[RFC] KVM: optimize the kvm_vcpu_on_spin

["Longpeng(Mike)" <longpeng2@huawei.com>]

We had disscuss the idea here:
https://www.spinics.net/lists/kvm/msg140593.html

I think it's also suitable for other architectures.

If the vcpu(me) exit due to request a usermode spinlock, then
the spinlock-holder may be preempted in usermode or kernmode.
But if the vcpu(me) is in kernmode, then the holder must be
preempted in kernmode, so we should choose a vcpu in kernmode
as the most eligible candidate.

PS: I only implement X86 arch currently for I'm not familiar
    with other architecture.

Signed-off-by: Longpeng(Mike) <longpeng2@huawei.com>
---
 arch/mips/kvm/mips.c       | 5 +++++
 arch/powerpc/kvm/powerpc.c | 5 +++++
 arch/s390/kvm/kvm-s390.c   | 5 +++++
 arch/x86/kvm/x86.c         | 5 +++++
 include/linux/kvm_host.h   | 4 ++++
 virt/kvm/arm/arm.c         | 5 +++++
 virt/kvm/kvm_main.c        | 9 ++++++++-
 7 files changed, 37 insertions(+), 1 deletion(-)

diff --git a/arch/mips/kvm/mips.c b/arch/mips/kvm/mips.c
index d4b2ad1..2e2701d 100644
--- a/arch/mips/kvm/mips.c
+++ b/arch/mips/kvm/mips.c
@@ -98,6 +98,11 @@ int kvm_arch_vcpu_runnable(struct kvm_vcpu *vcpu)
 	return !!(vcpu->arch.pending_exceptions);
 }
 
+bool kvm_arch_vcpu_spin_kernmode(struct kvm_vcpu *vcpu)
+{
+	return false;
+}
+
 int kvm_arch_vcpu_should_kick(struct kvm_vcpu *vcpu)
 {
 	return 1;
diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
index 1a75c0b..2489f64 100644
--- a/arch/powerpc/kvm/powerpc.c
+++ b/arch/powerpc/kvm/powerpc.c
@@ -58,6 +58,11 @@ int kvm_arch_vcpu_runnable(struct kvm_vcpu *v)
 	return !!(v->arch.pending_exceptions) || kvm_request_pending(v);
 }
 
+bool kvm_arch_vcpu_spin_kernmode(struct kvm_vcpu *vcpu)
+{
+	return false;
+}
+
 int kvm_arch_vcpu_should_kick(struct kvm_vcpu *vcpu)
 {
 	return 1;
diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index 3f2884e..9d7c42e 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -2443,6 +2443,11 @@ int kvm_arch_vcpu_runnable(struct kvm_vcpu *vcpu)
 	return kvm_s390_vcpu_has_irq(vcpu, 0);
 }
 
+bool kvm_arch_vcpu_spin_kernmode(struct kvm_vcpu *vcpu)
+{
+	return false;
+}
+
 void kvm_s390_vcpu_block(struct kvm_vcpu *vcpu)
 {
 	atomic_or(PROG_BLOCK_SIE, &vcpu->arch.sie_block->prog20);
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 82a63c5..b5a2e53 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8435,6 +8435,11 @@ int kvm_arch_vcpu_runnable(struct kvm_vcpu *vcpu)
 	return kvm_vcpu_running(vcpu) || kvm_vcpu_has_events(vcpu);
 }
 
+bool kvm_arch_vcpu_spin_kernmode(struct kvm_vcpu *vcpu)
+{
+	return kvm_x86_ops->get_cpl(vcpu) == 0;
+}
+
 int kvm_arch_vcpu_should_kick(struct kvm_vcpu *vcpu)
 {
 	return kvm_vcpu_exiting_guest_mode(vcpu) == IN_GUEST_MODE;
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 648b34c..f8f0d74 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -272,6 +272,9 @@ struct kvm_vcpu {
 	} spin_loop;
 #endif
 	bool preempted;
+	/* If vcpu is in kernel-mode when preempted */
+	bool in_kernmode;
+
 	struct kvm_vcpu_arch arch;
 	struct dentry *debugfs_dentry;
 };
@@ -797,6 +800,7 @@ int kvm_arch_vcpu_ioctl_set_guest_debug(struct kvm_vcpu *vcpu,
 void kvm_arch_hardware_unsetup(void);
 void kvm_arch_check_processor_compat(void *rtn);
 int kvm_arch_vcpu_runnable(struct kvm_vcpu *vcpu);
+bool kvm_arch_vcpu_spin_kernmode(struct kvm_vcpu *vcpu);
 int kvm_arch_vcpu_should_kick(struct kvm_vcpu *vcpu);
 
 #ifndef __KVM_HAVE_ARCH_VM_ALLOC
diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
index a39a1e1..ca6a394 100644
--- a/virt/kvm/arm/arm.c
+++ b/virt/kvm/arm/arm.c
@@ -416,6 +416,11 @@ int kvm_arch_vcpu_runnable(struct kvm_vcpu *v)
 		&& !v->arch.power_off && !v->arch.pause);
 }
 
+bool kvm_arch_vcpu_spin_kernmode(struct kvm_vcpu *vcpu)
+{
+	return false;
+}
+
 /* Just ensure a guest exit from a particular CPU */
 static void exit_vm_noop(void *info)
 {
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 82987d4..8d83caa 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -290,6 +290,7 @@ int kvm_vcpu_init(struct kvm_vcpu *vcpu, struct kvm *kvm, unsigned id)
 	kvm_vcpu_set_in_spin_loop(vcpu, false);
 	kvm_vcpu_set_dy_eligible(vcpu, false);
 	vcpu->preempted = false;
+	vcpu->in_kernmode = false;
 
 	r = kvm_arch_vcpu_init(vcpu);
 	if (r < 0)
@@ -2330,6 +2331,7 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me)
 	int pass;
 	int i;
 
+	me->in_kernmode = kvm_arch_vcpu_spin_kernmode(me);
 	kvm_vcpu_set_in_spin_loop(me, true);
 	/*
 	 * We boost the priority of a VCPU that is runnable but not
@@ -2351,6 +2353,8 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me)
 				continue;
 			if (swait_active(&vcpu->wq) && !kvm_arch_vcpu_runnable(vcpu))
 				continue;
+			if (me->in_kernmode && !vcpu->in_kernmode)
+				continue;
 			if (!kvm_vcpu_eligible_for_directed_yield(vcpu))
 				continue;
 
@@ -4009,8 +4013,11 @@ static void kvm_sched_out(struct preempt_notifier *pn,
 {
 	struct kvm_vcpu *vcpu = preempt_notifier_to_vcpu(pn);
 
-	if (current->state == TASK_RUNNING)
+	if (current->state == TASK_RUNNING) {
 		vcpu->preempted = true;
+		vcpu->in_kernmode = kvm_arch_vcpu_spin_kernmode(vcpu);
+	}
+
 	kvm_arch_vcpu_put(vcpu);
 }
 
-- 
1.8.3.1