linux-kernel - Re: [PATCH] kvm: don't take vcpu mutex for obviously invalid vcpu ioctls

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Date:	Mon, 22 Sep 2014 16:31:48 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Christian Borntraeger <borntraeger@...ibm.com>,
	David Matlack <dmatlack@...gle.com>,
	Gleb Natapov <gleb@...nel.org>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kvm: don't take vcpu mutex for obviously invalid vcpu
 ioctls

Il 22/09/2014 15:45, Christian Borntraeger ha scritto:
> We now have an extra condition check for every valid ioctl, to make an error case go faster.
> I know, the extra check is just a 1 or 2 cycles if branch prediction is right, but still.

I applied the patch because the delay could be substantial, depending on
what the other VCPU is doing.  Perhaps something like this would be
better?

(Untested, but Tested-by/Reviewed-bys are welcome).

Paolo

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 84e24b210273..ed31760d79fe 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -117,12 +117,10 @@ bool kvm_is_mmio_pfn(pfn_t pfn)
 /*
  * Switches to specified vcpu, until a matching vcpu_put()
  */
-int vcpu_load(struct kvm_vcpu *vcpu)
+static void __vcpu_load(struct kvm_vcpu *vcpu)
 {
 	int cpu;
 
-	if (mutex_lock_killable(&vcpu->mutex))
-		return -EINTR;
 	if (unlikely(vcpu->pid != current->pids[PIDTYPE_PID].pid)) {
 		/* The thread running this VCPU changed. */
 		struct pid *oldpid = vcpu->pid;
@@ -136,6 +134,14 @@ int vcpu_load(struct kvm_vcpu *vcpu)
 	preempt_notifier_register(&vcpu->preempt_notifier);
 	kvm_arch_vcpu_load(vcpu, cpu);
 	put_cpu();
+}
+
+int vcpu_load(struct kvm_vcpu *vcpu)
+{
+	if (mutex_lock_killable(&vcpu->mutex))
+		return -EINTR;
+
+	__vcpu_load(vcpu);
 	return 0;
 }
 
@@ -1989,9 +1995,6 @@ static long kvm_vcpu_ioctl(struct file *filp,
 	if (vcpu->kvm->mm != current->mm)
 		return -EIO;
 
-	if (unlikely(_IOC_TYPE(ioctl) != KVMIO))
-		return -EINVAL;
-
 #if defined(CONFIG_S390) || defined(CONFIG_PPC) || defined(CONFIG_MIPS)
 	/*
 	 * Special cases: vcpu ioctls that are asynchronous to vcpu execution,
@@ -2001,8 +2004,21 @@ static long kvm_vcpu_ioctl(struct file *filp,
 		return kvm_arch_vcpu_ioctl(filp, ioctl, arg);
 #endif
 
+	if (!mutex_trylock(&vcpu->mutex))) {
+		/*
+		 * Before a potentially long sleep, check if we'd exit anyway.
+		 * The common case is for the mutex not to be contended, when
+		 * this does not add overhead.
+		 */
+		if (unlikely(_IOC_TYPE(ioctl) != KVMIO))
+			return -EINVAL;
+
+		if (mutex_lock_killable(&vcpu->mutex))
+			return -EINTR;
+	}
+
 
-	r = vcpu_load(vcpu);
+	r = __vcpu_load(vcpu);
 	if (r)
 		return r;
 	switch (ioctl) {

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