On 2012-01-09 23:05, Alex Williamson wrote:
> On Mon, 2012-01-09 at 22:25 +0100, Jan Kiszka wrote:
>> On 2012-01-09 20:45, Alex Williamson wrote:
>>> On Mon, 2012-01-09 at 15:03 +0100, Jan Kiszka wrote:
>>>> +static int kvm_vm_ioctl_set_pci_irq_mask(struct kvm *kvm,
>>>> + struct kvm_assigned_pci_dev *assigned_dev)
>>>> +{
>>>> + int r = 0;
>>>> + struct kvm_assigned_dev_kernel *match;
>>>> +
>>>> + mutex_lock(&kvm->lock);
>>>> +
>>>> + match = kvm_find_assigned_dev(&kvm->arch.assigned_dev_head,
>>>> + assigned_dev->assigned_dev_id);
>>>> + if (!match) {
>>>> + r = -ENODEV;
>>>> + goto out;
>>>> + }
>>>> +
>>>> + mutex_lock(&match->intx_mask_lock);
>>>> +
>>>> + match->flags &= ~KVM_DEV_ASSIGN_MASK_INTX;
>>>> + match->flags |= assigned_dev->flags & KVM_DEV_ASSIGN_MASK_INTX;
>>>> +
>>>> + if (match->irq_requested_type & KVM_DEV_IRQ_GUEST_INTX) {
>>>> + if (assigned_dev->flags & KVM_DEV_ASSIGN_MASK_INTX) {
>>>> + kvm_set_irq(match->kvm, match->irq_source_id,
>>>> + match->guest_irq, 0);
>>>> + /*
>>>> + * Masking at hardware-level is performed on demand,
>>>> + * i.e. when an IRQ actually arrives at the host.
>>>> + */
>>>
>>> Is there any harm in doing this synchronous to the ioctl? We're on a
>>> slow path here anyway since the mask is likely drive by a config space
>>> write.
>>
>> Not sure, maybe locking. What would be the advantage of doing it
>> synchronously?
>
> It would just be a closer match to hardware. I'm wondering (FUD) if
> there could be a case where a driver does some sensitive operations on
> the device that could be interfered with if the device generates that
> one last interrupt to actually disable interrupts instead of them being
> disabled after setting config space.
The guest driver will never see such an interrupt as we will notice on
its arrival that there is some mask pending.
> It's probably a long shot, but
> doesn't seem too difficult to switch to synchronous disabling.
It is a bit as we have no PCI API in place to implement this. We only
have check-and-mask which does not mask if there is no IRQ raised. How
do you handle this in VFIO so far?
Really, I do not see an urgent need for synchronous masking and would
rather refrain from it until we are aware of a real problem with
asynchronous one as implemented here.
Jan
--
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux
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