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

[Matthew Rosato]

On 4/23/21 7:42 AM, Cornelia Huck wrote:
> 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: 20210419184906.2847283-1-farman@linux.ibm.com/">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

So the unregister really does 4 things of interest:

1) vfio_set_irq_signaling(VFIO_IRQ_SET_ACTION_TRIGGER)
This will drive VFIO_DEVICE_SET_IRQS ioctl, and it looks to me like the 
VFIO_DEVICE_SET_IRQS ioctl should return -EINVAL if it's not registered 
with the kernel, which will in turn cause the vfio_set_irq_signaling to 
fast-path out on a return 0.  That seems correct.

2) qemu_set_fd_handler
If we never registered (or it was already unregistered) then fd should 
not show up in find_aio_handler() so we should also bail out fast on 
qemu_set_fd_handler()

3) event_notifier_cleanup
Should bail out right away if already cleaned up before (!initialized)

So, this looks OK to me.


> 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.

But +1 on driving the path and making sure it works anyway (do a 
double-unregister?)