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From: Christian =?iso-8859-1?q?Borntr=E4ger?= <borntrae@de.ibm.com>
Subject: Re: [patch 0/4] move irq protection role to separate lock v2
Date: Thu, 21 May 2009 08:55:53 +0200
Message-ID: <200905210855.53470.borntrae@de.ibm.com>
References: <4A1413C3.4020606@redhat.com> <20090520184841.954066003@localhost.localdomain> <20090521045015.GA1104@amt.cnet>
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Cc: Avi Kivity <avi@redhat.com>, kvm@vger.kernel.org,
	Christian Borntraeger <borntraeger@de.ibm.com>
To: Marcelo Tosatti <mtosatti@redhat.com>,
	Christian Ehrhardt <ehrhardt@linux.vnet.ibm.com>,
	Carsten Otte <cotte@de.ibm.com>
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Am Donnerstag 21 Mai 2009 06:50:15 schrieb Marcelo Tosatti:
> But I fail to see the case where vcpu creation is a fast path (unless
> you're benchmarking cpu hotplug/hotunplug).

[...]

> @@ -2053,6 +2054,9 @@ static long kvm_vm_ioctl(struct file *fi
>
>  	if (kvm->mm != current->mm)
>  		return -EIO;
> +
> +	mutex_lock(&kvm->vm_ioctl_lock);
> +
>  	switch (ioctl) {
>  	case KVM_CREATE_VCPU:
>  		r = kvm_vm_ioctl_create_vcpu(kvm, arg);
> @@ -2228,6 +2232,7 @@ static long kvm_vm_ioctl(struct file *fi
>  		r = kvm_arch_vm_ioctl(filp, ioctl, arg);
>  	}
>  out:
> +	mutex_unlock(&kvm->vm_ioctl_lock);
>  	return r;
>  }

The thing that looks worrysome is that the s390 version of kvm_arch_vm_ioctl 
has KVM_S390_INTERRUPT. This allows userspace to inject interrupts - which 
would be serialized. The thing is, that external interrupts and I/O interrupts 
are floating - which means they can arrive on all cpus. This is somewhat of a 
fast path.
On the other hand, kvm_s390_inject_vm already takes the kvm->lock to protect 
agains hotplug. With this patch we might be able to remove the kvm->lock in 
kvm_s390_inject_vm - that would reduce the impact. 

This needs more thinking on our side.

Christian