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

Christian Bornträger
2009-05-21T06:55:54Z

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