linux-kernel - Re: [PATCH v3] virtio: add VIRTIO_F_NOTIFICATION

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Date:   Tue, 21 Mar 2023 18:22:31 +0300
From:   Viktor Prutyanov <viktor@...nix.com>
To:     Cornelia Huck <cohuck@...hat.com>
Cc:     mst@...hat.com, jasowang@...hat.com, pasic@...ux.ibm.com,
        farman@...ux.ibm.com, linux-s390@...r.kernel.org,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, yan@...nix.com
Subject: Re: [PATCH v3] virtio: add VIRTIO_F_NOTIFICATION_DATA feature support

On Tue, Mar 21, 2023 at 5:59 PM Cornelia Huck <cohuck@...hat.com> wrote:
>
> On Tue, Mar 21 2023, Viktor Prutyanov <viktor@...nix.com> wrote:
>
> > According to VirtIO spec v1.2, VIRTIO_F_NOTIFICATION_DATA feature
> > indicates that the driver passes extra data along with the queue
> > notifications.
> >
> > In a split queue case, the extra data is 16-bit available index. In a
> > packed queue case, the extra data is 1-bit wrap counter and 15-bit
> > available index.
> >
> > Add support for this feature for MMIO, PCI and channel I/O transports.
> >
> > Signed-off-by: Viktor Prutyanov <viktor@...nix.com>
> > ---
> >  v3: support feature in virtio_ccw, remove VM_NOTIFY, use avail_idx_shadow,
> >     remove byte swap, rename to vring_notification_data
> >  v2: reject the feature in virtio_ccw, replace __le32 with u32
> >
> >  drivers/s390/virtio/virtio_ccw.c   |  4 +++-
> >  drivers/virtio/virtio_mmio.c       | 14 +++++++++++++-
> >  drivers/virtio/virtio_pci_common.c | 10 ++++++++++
> >  drivers/virtio/virtio_pci_common.h |  4 ++++
> >  drivers/virtio/virtio_pci_legacy.c |  2 +-
> >  drivers/virtio/virtio_pci_modern.c |  2 +-
> >  drivers/virtio/virtio_ring.c       | 17 +++++++++++++++++
> >  include/linux/virtio_ring.h        |  2 ++
> >  include/uapi/linux/virtio_config.h |  6 ++++++
> >  9 files changed, 57 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/s390/virtio/virtio_ccw.c b/drivers/s390/virtio/virtio_ccw.c
> > index 954fc31b4bc7..c33172c5b8d5 100644
> > --- a/drivers/s390/virtio/virtio_ccw.c
> > +++ b/drivers/s390/virtio/virtio_ccw.c
> > @@ -396,13 +396,15 @@ static bool virtio_ccw_kvm_notify(struct virtqueue *vq)
> >       struct virtio_ccw_vq_info *info = vq->priv;
> >       struct virtio_ccw_device *vcdev;
> >       struct subchannel_id schid;
> > +     u32 data = __virtio_test_bit(vq->vdev, VIRTIO_F_NOTIFICATION_DATA) ?
> > +                     vring_notification_data(vq) : vq->index;
> >
> >       vcdev = to_vc_device(info->vq->vdev);
> >       ccw_device_get_schid(vcdev->cdev, &schid);
> >       BUILD_BUG_ON(sizeof(struct subchannel_id) != sizeof(unsigned int));
> >       info->cookie = kvm_hypercall3(KVM_S390_VIRTIO_CCW_NOTIFY,
> >                                     *((unsigned int *)&schid),
> > -                                   vq->index, info->cookie);
> > +                                   data, info->cookie);
> >       if (info->cookie < 0)
> >               return false;
> >       return true;
> > diff --git a/drivers/virtio/virtio_mmio.c b/drivers/virtio/virtio_mmio.c
> > index 3ff746e3f24a..7c16e622c33d 100644
> > --- a/drivers/virtio/virtio_mmio.c
> > +++ b/drivers/virtio/virtio_mmio.c
> > @@ -285,6 +285,16 @@ static bool vm_notify(struct virtqueue *vq)
> >       return true;
> >  }
> >
> > +static bool vm_notify_with_data(struct virtqueue *vq)
> > +{
> > +     struct virtio_mmio_device *vm_dev = to_virtio_mmio_device(vq->vdev);
> > +     u32 data = vring_notification_data(vq);
> > +
> > +     writel(data, vm_dev->base + VIRTIO_MMIO_QUEUE_NOTIFY);
>
> Can't you simply use the same method as for ccw, i.e. use one callback
> function that simply writes one value or the other?

The idea is to eliminate the conditional branch induced by feature bit
testing from the notification function. Probably, this can be done in
the same way in ccw.

>
> > +
> > +     return true;
> > +}
> > +
> >  /* Notify all virtqueues on an interrupt. */
> >  static irqreturn_t vm_interrupt(int irq, void *opaque)
> >  {
> > @@ -368,6 +378,8 @@ static struct virtqueue *vm_setup_vq(struct virtio_device *vdev, unsigned int in
> >       unsigned long flags;
> >       unsigned int num;
> >       int err;
> > +     bool (*notify)(struct virtqueue *vq) = __virtio_test_bit(vdev,
> > +             VIRTIO_F_NOTIFICATION_DATA) ? vm_notify_with_data : vm_notify;
> >
> >       if (!name)
> >               return NULL;
> > @@ -397,7 +409,7 @@ static struct virtqueue *vm_setup_vq(struct virtio_device *vdev, unsigned int in
> >
> >       /* Create the vring */
> >       vq = vring_create_virtqueue(index, num, VIRTIO_MMIO_VRING_ALIGN, vdev,
> > -                              true, true, ctx, vm_notify, callback, name);
> > +                              true, true, ctx, notify, callback, name);
> >       if (!vq) {
> >               err = -ENOMEM;
> >               goto error_new_virtqueue;
> > diff --git a/drivers/virtio/virtio_pci_common.c b/drivers/virtio/virtio_pci_common.c
> > index a6c86f916dbd..e915c22f2384 100644
> > --- a/drivers/virtio/virtio_pci_common.c
> > +++ b/drivers/virtio/virtio_pci_common.c
> > @@ -43,6 +43,16 @@ bool vp_notify(struct virtqueue *vq)
> >       /* we write the queue's selector into the notification register to
> >        * signal the other end */
> >       iowrite16(vq->index, (void __iomem *)vq->priv);
> > +
> > +     return true;
> > +}
> > +
> > +bool vp_notify_with_data(struct virtqueue *vq)
> > +{
> > +     u32 data = vring_notification_data(vq);
> > +
> > +     iowrite32(data, (void __iomem *)vq->priv);
>
> Same for pci.
>
> > +
> >       return true;
> >  }
> >
>