Re: [PATCH v4 9/9] KVM: Allow host IRQ sharing for passed-through PCI 2.3 devices

Avi Kivity
2010-11-09T13:41:20Z

On 11/09/2010 03:35 PM, Jan Kiszka wrote:
> Am 09.11.2010 14:27, Avi Kivity wrote:
> > On 11/08/2010 01:21 PM, Jan Kiszka wrote:
> >> PCI 2.3 allows to generically disable IRQ sources at device level. This
> >> enables us to share IRQs of such devices between on the host side when
> >> passing them to a guest. This feature is optional, user space has to
> >> request it explicitly. Moreover, user space can inform us about its view
> >> of PCI_COMMAND_INTX_DISABLE so that we can avoid unmasking the interrupt
> >> and signaling it if the guest masked it via the PCI config space.
> >>
> >
> > It's a pity this cannot be done transparently. We could detect multiple
> > devices sharing the line,
>
> Even that is not possible. Assigned or host devices may be activated
> after we registered exclusively, pushing the breakage from VM start-up
> to a different operation.

We could detect that and switch the interrupt mode. Or we could always
to IRQF_SHARED and fake something in the immediate callback.

> > but what about PCI_COMMAND_INTX_DISABLE?
> >
> > Perhaps we can hook the kernel's handler for this bit?
>
> Some IRQ registration notifier that would allow us to reregister our
> handler with IRQ sharing support? Maybe.

Adding an internal API if preferable to an external one (it may be a
pain to kvm-kmod users though).

> >
> >> +
> >> +Capability: KVM_CAP_PCI_2_3
> >> +Architectures: x86
> >> +Type: vm ioctl
> >> +Parameters: struct kvm_assigned_pci_dev (in)
> >> +Returns: 0 on success, -1 on error
> >> +
> >> +Informs the kernel about the guest's view on the INTx mask. As long as the
> >> +guest masks the legacy INTx, the kernel will refrain from unmasking it at
> >> +hardware level and will not assert the guest's IRQ line. User space is still
> >> +responsible for applying this state to the assigned device's real config space.
> >
> > What if userspace lies?
>
> User space problem. We will at worst receive one IRQ, mask it, and then
> user space need to react again.

Ok.

> >
> > I saw no reason this can't be a spinlock, but perhaps I missed
> > something. This would allow us to avoid srcu, which is slightly more
> > expensive than rcu. Since pci 2.3 assigned devices are not a major use
> > case, I'd like not to penalize the mainstream users for this.
>
> The lock has to be held across kvm_set_irq, which is the potentially
> expensive (O(n), n == number of VCPUs) operation.

What we should probably do is have broadcast interrupts deferred to a
thread. I agree it isn't pretty.

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