Re: [Qemu-arm] [PATCH V2] virtio: Fix no interrupt when not creating msi controller

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jan 24, 2017 at 04:00:55PM +0100, Paolo Bonzini wrote:
> 
> 
> On 24/01/2017 15:44, Michael S. Tsirkin wrote:
> > On Tue, Jan 24, 2017 at 10:25:00AM +0100, Paolo Bonzini wrote:
> >>
> >>
> >> On 23/01/2017 18:04, Michael S. Tsirkin wrote:
> >>>> For virtio-blk, my patch moved the setting of ISR from
> >>>> virtio_queue_guest_notifier_read to virtio_notify_irqfd.  This is
> >>>> because the irqfd emulation only needs to trigger the interrupt.
> >>>> Setting the ISR should have been done elsewhere.
> >>>>
> >>>> Can vhost set ISR (I thought it couldn't)?  If so, it seems like ARM was
> >>>> relying on QEMU's irqfd emulation, as a work around for vhost not
> >>>> setting ISR.  But this only works if irqfd is directed to QEMU and not
> >>>> to KVM.  So if vhost cannot set ISR, I think vhost should be disabled
> >>>> unless MSI is active.
> >>>>
> >>>> Am I missing something?
> >>>
> >>> vhost doesn't set ISR ATM.  Without MSI we are always bouncing
> >>> interrupts through QEMU in particular in order to set ISR.
> >>
> >> Where is the code that disables KVM irqfd?
> >
> > This bit in virtio_pci_set_guest_notifiers does the trick I think:
> > 
> > 
> >     bool with_irqfd = msix_enabled(&proxy->pci_dev) &&
> >         kvm_msi_via_irqfd_enabled();
> > 
> > I don't think we ever used irqfd for level interrupts.
> 
> Ok, thanks!
> 
> So we have four cases:
> 
> - assign with emulated irqfd + ISR
> - assign with emulated irqfd
> - assign with irqfd
> - deassign
> 
> Currently we don't distinguish the first two, so (which I don't like)
> Shannon's patch ends up setting ISR twice.

I don't see this yet - where is it set twice?

> The only (ugly) solution
> that comes to mind is making with_irqfd an enum...
> 
> Paolo