On Mon, 2011-03-14 at 21:00 +0200, Michael S. Tsirkin wrote:
> On Mon, Mar 14, 2011 at 10:35:08PM +0530, rukhsana ansari wrote:
> > Seeking clarification to the original question I posted:
> > >>
> > >>
> > > This maybe a novice question - Would appreciate it if you can you provide a
> > > pointer to documentation or relevant code that explains what is the
> > > limitation in supporting level irq support in kvm irqfd.
> > >
> > >
> > >
> > After browsing the KVM kernel code, it does look like direct assignment of PCI
> > devices allows support for level-triggered interrupts to be injected to the
> > guest from the kernel. (as opposed to not supporting it for vhost irqfd
> > mechanism)
> > This occurs when the guest device supports INTX.
> > Reference: kvm_assigned_dev_interrupt_work_handler() in assigned-dev.c calls
> > kvm_set_irq()
> > with the guest_irq.
> > This function in turn invokes the assigned set function (either
> > kvm_set_pic_irq or kvm_set_ioapic_irq) which was setup at kvm_irq_chip creation
> > time when kvm_setup_default_irq_routing () called for handling ioctl
> > KVM_CREATE_IRQCHIP.
> >
> > So, it isn't clear why level-triggered interrupt isn't supported for irqfd
> > mechanism.
> > Would greatly appreciate clarification here
> >
> > Thanks
> > -Rukhsana
> >
>
> Mostly, no one came up with an implementation so far.
>
> If the point is to use irqfd with vhost-net, there's also
> a question of adding interfaces to
> 1. pass IO read transactions directly to another kernel module
> 2. add an interface to clear the irq level
>
> Maybe the right thing is to combine the two somehow:
> irqfd might get an oiption to set a bit in memory,
> ioeventfd might get an option to read and clear from memory
> and clear irqfd line at the same time.