[PATCH v2 2/3] KVM: send IPI to vcpu only when it's in guest mode

[Xiao Guangrong]

We can interrupt the vcpu only when it's running in guest mode
to reduce IPI

It looks like only ia64 and x86 need to send IPI to other vcpus, so
i only add the implementation of 'vcpu->guest_mode' in ia64, but i
don't know ia64 well, please point out the right way for me if the
implementation is incorrect

Also reorganize struct kvm_vcpu to make ->guest_mode and ->requests
in the same cache line explicitly.

Signed-off-by: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxx>
---
 arch/ia64/kvm/kvm-ia64.c |    2 ++
 arch/x86/kvm/x86.c       |    6 +++++-
 include/linux/kvm_host.h |    9 +++++----
 virt/kvm/kvm_main.c      |    7 ++++++-
 4 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/arch/ia64/kvm/kvm-ia64.c b/arch/ia64/kvm/kvm-ia64.c
index 70d224d..15c11b2 100644
--- a/arch/ia64/kvm/kvm-ia64.c
+++ b/arch/ia64/kvm/kvm-ia64.c
@@ -662,6 +662,7 @@ again:
 		goto vcpu_run_fail;
 
 	srcu_read_unlock(&vcpu->kvm->srcu, idx);
+	atomic_set(&vcpu->guest_mode, 1);
 	kvm_guest_enter();
 
 	/*
@@ -683,6 +684,7 @@ again:
 	 */
 	barrier();
 	kvm_guest_exit();
+	atomic_set(&vcpu->guest_mode, 0);
 	preempt_enable();
 
 	idx = srcu_read_lock(&vcpu->kvm->srcu);
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 0b77e9c..f882158 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5214,7 +5214,11 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 	kvm_load_guest_xcr0(vcpu);
 
 	atomic_set(&vcpu->guest_mode, 1);
-	smp_wmb();
+
+	/* We should set ->guest_mode before check ->requests,
+	 * see the comment in make_all_cpus_request.
+	 */
+	smp_mb();
 
 	local_irq_disable();
 
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index b5021db..5f5ef74 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -103,14 +103,15 @@ struct kvm_vcpu {
 #ifdef CONFIG_PREEMPT_NOTIFIERS
 	struct preempt_notifier preempt_notifier;
 #endif
+	int cpu;
 	int vcpu_id;
-	struct mutex mutex;
-	int   cpu;
+	int srcu_idx;
 	atomic_t guest_mode;
-	struct kvm_run *run;
 	unsigned long requests;
 	unsigned long guest_debug;
-	int srcu_idx;
+
+	struct mutex mutex;
+	struct kvm_run *run;
 
 	int fpu_active;
 	int guest_fpu_loaded, guest_xcr0_loaded;
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index b1b6cbb..a475264 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -153,7 +153,12 @@ static bool make_all_cpus_request(struct kvm *kvm, unsigned int req)
 		if (kvm_make_check_request(req, vcpu))
 			continue;
 		cpu = vcpu->cpu;
-		if (cpus != NULL && cpu != -1 && cpu != me)
+
+		/* Set ->requests bit before we read ->guest_mode */
+		smp_mb();
+
+		if (cpus != NULL && cpu != -1 && cpu != me &&
+		      atomic_read(&vcpu->guest_mode))
 			cpumask_set_cpu(cpu, cpus);
 	}
 	if (unlikely(cpus == NULL))
-- 
1.7.3.4

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