Re: [RFC PATCH] vfio-ccw: Permit missing IRQs

[Cornelia Huck <cohuck@redhat.com>]

On Mon, 19 Apr 2021 20:49:06 +0200
Eric Farman <farman@linux.ibm.com> wrote:

> Commit 690e29b91102 ("vfio-ccw: Refactor ccw irq handler") changed
> one of the checks for the IRQ notifier registration from saying
> "the host needs to recognize the only IRQ that exists" to saying
> "the host needs to recognize ANY IRQ that exists."
> 
> And this worked fine, because the subsequent change to support the
> CRW IRQ notifier doesn't get into this code when running on an older
> kernel, thanks to a guard by a capability region. The later addition
> of the REQ(uest) IRQ by commit b2f96f9e4f5f ("vfio-ccw: Connect the
> device request notifier") broke this assumption because there is no
> matching capability region. Thus, running new QEMU on an older
> kernel fails with:
> 
>   vfio: unexpected number of irqs 2
> 
> Let's simply remove the check (and the less-than-helpful message),
> and make the VFIO_DEVICE_GET_IRQ_INFO ioctl request for the IRQ
> being processed. If it returns with EINVAL, we can treat it as
> an unfortunate mismatch but not a fatal error for the guest.
> 
> Fixes: 690e29b91102 ("vfio-ccw: Refactor ccw irq handler")
> Fixes: b2f96f9e4f5f ("vfio-ccw: Connect the device request notifier")
> Signed-off-by: Eric Farman <farman@linux.ibm.com>
> ---
>  hw/vfio/ccw.c | 15 +++++++--------
>  1 file changed, 7 insertions(+), 8 deletions(-)
> 
> diff --git a/hw/vfio/ccw.c b/hw/vfio/ccw.c
> index b2df708e4b..cfbfc3d1a2 100644
> --- a/hw/vfio/ccw.c
> +++ b/hw/vfio/ccw.c
> @@ -411,20 +411,19 @@ static void vfio_ccw_register_irq_notifier(VFIOCCWDevice *vcdev,
>          return;
>      }
>  
> -    if (vdev->num_irqs < irq + 1) {
> -        error_setg(errp, "vfio: unexpected number of irqs %u",
> -                   vdev->num_irqs);

Alternative proposal: Change this message to

"vfio: IRQ %u not available (number of irqs %u)"

and still fail this function, while treating a failure of
vfio_ccw_register_irq_notifier(vcdev, VFIO_CCW_REQ_IRQ_INDEX, &err); in
vfio_ccw_realize() as a non-fatal error (maybe log a message).

This allows us to skip doing an ioctl call, of which we already know
that it would fail. Still, we can catch cases where a broken kernel e.g.
provides the crw region, but not the matching irq (I believe something
like that should indeed be a fatal error.)

> -        return;
> -    }
> -
>      argsz = sizeof(*irq_info);
>      irq_info = g_malloc0(argsz);
>      irq_info->index = irq;
>      irq_info->argsz = argsz;
>      if (ioctl(vdev->fd, VFIO_DEVICE_GET_IRQ_INFO,
>                irq_info) < 0 || irq_info->count < 1) {
> -        error_setg_errno(errp, errno, "vfio: Error getting irq info");
> -        goto out_free_info;
> +        if (errno == EINVAL) {
> +            warn_report("Unable to get information about IRQ %u", irq);
> +            goto out_free_info;
> +        } else {
> +            error_setg_errno(errp, errno, "vfio: Error getting irq info");
> +            goto out_free_info;
> +        }
>      }
>  
>      if (event_notifier_init(notifier, 0)) {