Re: [PATCH v2] vfio-ccw: Permit missing IRQs

[Cornelia Huck <cohuck@redhat.com>]

On Wed, 21 Apr 2021 17:20:53 +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 adapt the message here so that there's a better clue of what
> IRQ is missing.
> 
> Furthermore, let's make the REQ(uest) IRQ not fail when attempting
> to register it, to permit running vfio-ccw on a newer QEMU with an
> older kernel.
> 
> Fixes: b2f96f9e4f5f ("vfio-ccw: Connect the device request notifier")
> Signed-off-by: Eric Farman <farman@linux.ibm.com>
> ---
> 
> Notes:
>     v1->v2:
>      - Keep existing "invalid number of IRQs" message with adapted text [CH]
>      - Move the "is this an error" test to the registration of the IRQ in
>        question, rather than making it allowable for any IRQ mismatch [CH]
>      - Drop Fixes tag for initial commit [EF]
>     
>     v1: https://lore.kernel.org/qemu-devel/20210419184906.2847283-1-farman@linux.ibm.com/
> 
>  hw/vfio/ccw.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/hw/vfio/ccw.c b/hw/vfio/ccw.c
> index b2df708e4b..400bc07fe2 100644
> --- a/hw/vfio/ccw.c
> +++ b/hw/vfio/ccw.c
> @@ -412,8 +412,8 @@ static void vfio_ccw_register_irq_notifier(VFIOCCWDevice *vcdev,
>      }
>  
>      if (vdev->num_irqs < irq + 1) {
> -        error_setg(errp, "vfio: unexpected number of irqs %u",
> -                   vdev->num_irqs);
> +        error_setg(errp, "vfio: IRQ %u not available (number of irqs %u)",
> +                   irq, vdev->num_irqs);
>          return;
>      }
>  
> @@ -696,13 +696,15 @@ static void vfio_ccw_realize(DeviceState *dev, Error **errp)
>  
>      vfio_ccw_register_irq_notifier(vcdev, VFIO_CCW_REQ_IRQ_INDEX, &err);
>      if (err) {
> -        goto out_req_notifier_err;
> +        /*
> +         * Report this error, but do not make it a failing condition.
> +         * Lack of this IRQ in the host does not prevent normal operation.
> +         */
> +        error_report_err(err);
>      }
>  
>      return;
>  
> -out_req_notifier_err:
> -    vfio_ccw_unregister_irq_notifier(vcdev, VFIO_CCW_CRW_IRQ_INDEX);
>  out_crw_notifier_err:
>      vfio_ccw_unregister_irq_notifier(vcdev, VFIO_CCW_IO_IRQ_INDEX);
>  out_io_notifier_err:

Patch looks good to me, but now I'm wondering: Is calling
vfio_ccw_unregister_irq_notifier() for an unregistered irq actually
safe? I think it is (our notifiers are always present, and we should
handle any ioctl failure gracefully), but worth a second look. In fact,
we already unregister the crw irq unconditionally, so we would likely
already have seen any problems for that one, so I assume all is good.