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

[Avi Kivity <avi@redhat.com>]

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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