Re: [PATCH v4 08/23] virtio: move common part of _set_guest_notifier to generic code

[Halil Pasic <pasic@linux.ibm.com>]

On Mon, 26 Jan 2026 19:40:09 +0300
Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru> wrote:

> >> +/**
> >> + * virtio_queue_set_guest_notifier - set/unset queue or config guest
> >> + *     notifier
> >> + *
> >> + * @vdev: the VirtIO device
> >> + * @n: queue number, or VIRTIO_CONFIG_IRQ_IDX to set config notifer  
> > 
> > Semantically are mixing queue numbers and interrupt indexes here.  
> 
> That's preexisting logic in virtio_pci_set_guest_notifier_fd_handler().
> 
> But, you are right, that's doesn't answer the question, is it good to share
> the logic with mmio and ccw.
> 
> Is config notifier only pci specific, or similar feature may appear in future
> for mmio and/or ccw?

AFAIR there is no  notifier for ccw. A possible explanation for
that is, that AFAIK that config space notifications (device -> driver)
are always classic I/O interrupts while queue indicators can be so called
adapter I/O interrupts and irqfd is only available for adapter I/O
interrupts. 

If you like you can have a look at 
"4.3.2.6 Setting Up Indicators" in
https://docs.oasis-open.org/virtio/virtio/v1.3/csd01/virtio-v1.3-csd01.html#x1-1940003

I see no reason why virtio_config_guest_notifier_read() based, i.e
!irqfd virtio config notifier should not exist or would not work. And I
guess it is needed for certain vhost things. But nevertheless, I can't
find the place where a virtio-ccw config notifier is set up.

Eric, what is your opinion on this? My current thinking is that
virtio-ccw not having a config notifier is basically a bug.

Regards,
Halil