Re: [patch 0/4] move irq protection role to separate lock v2

["Christian Bornträger" <borntrae@de.ibm.com>]

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