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Date:   Fri, 1 Jul 2022 02:14:02 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Jason Wang <jasowang@redhat.com>
Cc:     Alexander Atanasov <alexander.atanasov@virtuozzo.com>,
        kernel@openvz.org, Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Marc Zyngier <maz@kernel.org>,
        Halil Pasic <pasic@linux.ibm.com>,
        Cornelia Huck <cohuck@redhat.com>,
        Vineeth Vijayan <vneethv@linux.ibm.com>,
        Peter Oberparleiter <oberpar@linux.ibm.com>,
        linux-s390@vger.kernel.org, Xuan Zhuo <xuanzhuo@linux.alibaba.com>,
        virtualization <virtualization@lists.linux-foundation.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v1 1/1] virtio: Restore semantics of vq->broken in
 virtqueues
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On Fri, Jul 01, 2022 at 09:12:58AM +0800, Jason Wang wrote:
> On Thu, Jun 30, 2022 at 11:44 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Thu, Jun 30, 2022 at 01:08:53PM +0300, Alexander Atanasov wrote:
> > > Hello,
> > >
> > > On 30/06/2022 12:46, Michael S. Tsirkin wrote:
> > > > On Thu, Jun 30, 2022 at 09:36:46AM +0000, Alexander Atanasov wrote:
> > > > > virtio: harden vring IRQ (8b4ec69d7e09) changed the use
> > > > > of vq->broken. As result vring_interrupt handles IRQs for
> > > > > broken drivers as IRQ_NONE and not IRQ_HANDLED and made impossible
> > > > > to initiallize vqs before the driver is ready, i.e. in probe method.
> > > > > Balloon driver does this and it can not load because it fails in
> > > > > vqs_init with -EIO.
> > > > >
> > > > > So instead of changing the original intent ot the flag introduce
> > > > > a new flag vq->ready which servers the purpose to check of early IRQs
> > > > > and restore the behaviour of the vq->broken flag.
> > > > >
> > > > > Signed-off-by: Alexander Atanasov <alexander.atanasov@virtuozzo.com>
> > > >
> > > > Does
> > > >
> > > > commit c346dae4f3fbce51bbd4f2ec5e8c6f9b91e93163
> > > > Author: Jason Wang <jasowang@redhat.com>
> > > > Date:   Wed Jun 22 09:29:40 2022 +0800
> > > >
> > > >      virtio: disable notification hardening by default
> > > >
> > > >
> > > > solve the problem for you?
> > >
> > >
> > > No, it won't if CONFIG_VIRTIO_HARDEN_NOTIFICATION is enabled - balloon still
> > > won't be able to init vqs.
> >
> > Yea I intend to make CONFIG_VIRTIO_HARDEN_NOTIFICATION
> > depend on BROKEN for now.
> >
> > > The problem is in virtqueue_add_split and virtqueue_add_packed - can not set
> > > driver_ok without queues.
> > >
> > > The return value of the vring_interrupt gets different - and iirc IRQ_NONE
> > > for broken device can lead to interrupt storms - i am not sure if that is
> > > valid for virtio devices yet but for real harware most likely.
> >
> > No, I think it's the reverse. With IRQ_HANDLED an interrupt
> > storm will keep overloading the CPU since driver tells
> > kernel all is well. With IRQ_NONE kernel will eventually
> > intervene and disable the irq.
> 
> Yes, and users may get a warn.
> 
> For IRQ_HANDLED, it has an issue when the driver is sharing IRQ with
> other drivers.
> 
> Thanks


Couldn't tell whether you are agreeing or disagreeing with me here.

> >
> > > Either way if
> > > you have a mix of  drivers working differently depending on return of the
> > > handler  it would get really messy.
> > >
> > > RR's original intent was to flag a driver as bad why reuse it like that  ?
> > >
> > >
> > > > >   drivers/virtio/virtio_ring.c  | 20 ++++++++++++++------
> > > > >   include/linux/virtio.h        |  2 +-
> > > > >   include/linux/virtio_config.h | 10 +++++-----
> > > > >   3 files changed, 20 insertions(+), 12 deletions(-)
> > > > >
> > > > > Cc: Thomas Gleixner <tglx@linutronix.de>
> > > > > Cc: Peter Zijlstra <peterz@infradead.org>
> > > > > Cc: "Paul E. McKenney" <paulmck@kernel.org>
> > > > > Cc: Marc Zyngier <maz@kernel.org>
> > > > > Cc: Halil Pasic <pasic@linux.ibm.com>
> > > > > Cc: Cornelia Huck <cohuck@redhat.com>
> > > > > Cc: Vineeth Vijayan <vneethv@linux.ibm.com>
> > > > > Cc: Peter Oberparleiter <oberpar@linux.ibm.com>
> > > > > Cc: linux-s390@vger.kernel.org
> > > > > Cc: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> > > > >
> > > > >
> > > > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > > > > index 13a7348cedff..dca3cc774584 100644
> > > > > --- a/drivers/virtio/virtio_ring.c
> > > > > +++ b/drivers/virtio/virtio_ring.c
> > > > > @@ -100,6 +100,9 @@ struct vring_virtqueue {
> > > > >           /* Other side has made a mess, don't try any more. */
> > > > >           bool broken;
> > > > > + /* the queue is ready to handle interrupts  */
> > > > > + bool ready;
> > > > > +
> > > > >           /* Host supports indirect buffers */
> > > > >           bool indirect;
> > > > > @@ -1688,7 +1691,8 @@ static struct virtqueue *vring_create_virtqueue_packed(
> > > > >           vq->we_own_ring = true;
> > > > >           vq->notify = notify;
> > > > >           vq->weak_barriers = weak_barriers;
> > > > > - vq->broken = true;
> > > > > + vq->broken = false;
> > > > > + vq->ready = false;
> > > > >           vq->last_used_idx = 0;
> > > > >           vq->event_triggered = false;
> > > > >           vq->num_added = 0;
> > > > > @@ -2134,7 +2138,10 @@ irqreturn_t vring_interrupt(int irq, void *_vq)
> > > > >                   return IRQ_NONE;
> > > > >           }
> > > > > - if (unlikely(vq->broken)) {
> > > > > + if (unlikely(vq->broken))
> > > > > +         return IRQ_HANDLED;
> > > > > +
> > > > > + if (unlikely(!vq->ready)) {
> > > > >                   dev_warn_once(&vq->vq.vdev->dev,
> > > > >                                 "virtio vring IRQ raised before DRIVER_OK");
> > > > >                   return IRQ_NONE;
> > > > > @@ -2180,7 +2187,8 @@ struct virtqueue *__vring_new_virtqueue(unsigned int index,
> > > > >           vq->we_own_ring = false;
> > > > >           vq->notify = notify;
> > > > >           vq->weak_barriers = weak_barriers;
> > > > > - vq->broken = true;
> > > > > + vq->broken = false;
> > > > > + vq->ready = false;
> > > > >           vq->last_used_idx = 0;
> > > > >           vq->event_triggered = false;
> > > > >           vq->num_added = 0;
> > > > > @@ -2405,7 +2413,7 @@ EXPORT_SYMBOL_GPL(virtio_break_device);
> > > > >    * (probing and restoring). This function should only be called by the
> > > > >    * core, not directly by the driver.
> > > > >    */
> > > > > -void __virtio_unbreak_device(struct virtio_device *dev)
> > > > > +void __virtio_device_ready(struct virtio_device *dev)
> > > > >   {
> > > > >           struct virtqueue *_vq;
> > > > > @@ -2414,11 +2422,11 @@ void __virtio_unbreak_device(struct virtio_device *dev)
> > > > >                   struct vring_virtqueue *vq = to_vvq(_vq);
> > > > >                   /* Pairs with READ_ONCE() in virtqueue_is_broken(). */
> > > > > -         WRITE_ONCE(vq->broken, false);
> > > > > +         WRITE_ONCE(vq->ready, true);
> > > > >           }
> > > > >           spin_unlock(&dev->vqs_list_lock);
> > > > >   }
> > > > > -EXPORT_SYMBOL_GPL(__virtio_unbreak_device);
> > > > > +EXPORT_SYMBOL_GPL(__virtio_device_ready);
> > > > >   dma_addr_t virtqueue_get_desc_addr(struct virtqueue *_vq)
> > > > >   {
> > > > > diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> > > > > index d8fdf170637c..538c5959949a 100644
> > > > > --- a/include/linux/virtio.h
> > > > > +++ b/include/linux/virtio.h
> > > > > @@ -131,7 +131,7 @@ void unregister_virtio_device(struct virtio_device *dev);
> > > > >   bool is_virtio_device(struct device *dev);
> > > > >   void virtio_break_device(struct virtio_device *dev);
> > > > > -void __virtio_unbreak_device(struct virtio_device *dev);
> > > > > +void __virtio_device_ready(struct virtio_device *dev);
> > > > >   void virtio_config_changed(struct virtio_device *dev);
> > > > >   #ifdef CONFIG_PM_SLEEP
> > > > > diff --git a/include/linux/virtio_config.h b/include/linux/virtio_config.h
> > > > > index 49c7c32815f1..35cf1b26e05a 100644
> > > > > --- a/include/linux/virtio_config.h
> > > > > +++ b/include/linux/virtio_config.h
> > > > > @@ -259,21 +259,21 @@ void virtio_device_ready(struct virtio_device *dev)
> > > > >           /*
> > > > >            * The virtio_synchronize_cbs() makes sure vring_interrupt()
> > > > > -  * will see the driver specific setup if it sees vq->broken
> > > > > +  * will see the driver specific setup if it sees vq->ready
> > > > >            * as false (even if the notifications come before DRIVER_OK).
> > > > >            */
> > > > >           virtio_synchronize_cbs(dev);
> > > > > - __virtio_unbreak_device(dev);
> > > > > + __virtio_device_ready(dev);
> > > > >           /*
> > > > > -  * The transport should ensure the visibility of vq->broken
> > > > > +  * The transport should ensure the visibility of vq->ready
> > > > >            * before setting DRIVER_OK. See the comments for the transport
> > > > >            * specific set_status() method.
> > > > >            *
> > > > >            * A well behaved device will only notify a virtqueue after
> > > > >            * DRIVER_OK, this means the device should "see" the coherenct
> > > > > -  * memory write that set vq->broken as false which is done by
> > > > > +  * memory write that set vq->ready as true which is done by
> > > > >            * the driver when it sees DRIVER_OK, then the following
> > > > > -  * driver's vring_interrupt() will see vq->broken as false so
> > > > > +  * driver's vring_interrupt() will see vq->true as true so
> > > > >            * we won't lose any notification.
> > > > >            */
> > > > >           dev->config->set_status(dev, status | VIRTIO_CONFIG_S_DRIVER_OK);
> > > > > --
> > > > > 2.25.1
> > >
> > > --
> > > Regards,
> > > Alexander Atanasov
> >