From:   Cornelia Huck <cohuck@redhat.com>
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     Viktor Prutyanov <viktor@daynix.com>, jasowang@redhat.com,
        pasic@linux.ibm.com, farman@linux.ibm.com,
        linux-s390@vger.kernel.org, kvm@vger.kernel.org,
        virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, yan@daynix.com
Subject: Re: [PATCH v3] virtio: add VIRTIO_F_NOTIFICATION_DATA feature support
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Organization: Red Hat GmbH
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Date:   Tue, 21 Mar 2023 17:21:59 +0100
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On Tue, Mar 21 2023, "Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Tue, Mar 21, 2023 at 04:30:57PM +0100, Cornelia Huck wrote:
>> On Tue, Mar 21 2023, Viktor Prutyanov <viktor@daynix.com> wrote:
>>=20
>> > On Tue, Mar 21, 2023 at 5:59=E2=80=AFPM Cornelia Huck <cohuck@redhat.c=
om> wrote:
>> >>
>> >> On Tue, Mar 21 2023, Viktor Prutyanov <viktor@daynix.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 transpor=
ts.
>> >> >
>> >> > Signed-off-by: Viktor Prutyanov <viktor@daynix.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 virt=
queue *vq)
>> >> >       struct virtio_ccw_vq_info *info =3D vq->priv;
>> >> >       struct virtio_ccw_device *vcdev;
>> >> >       struct subchannel_id schid;
>> >> > +     u32 data =3D __virtio_test_bit(vq->vdev, VIRTIO_F_NOTIFICATIO=
N_DATA) ?
>> >> > +                     vring_notification_data(vq) : vq->index;
>> >> >
>> >> >       vcdev =3D to_vc_device(info->vq->vdev);
>> >> >       ccw_device_get_schid(vcdev->cdev, &schid);
>> >> >       BUILD_BUG_ON(sizeof(struct subchannel_id) !=3D sizeof(unsigne=
d int));
>> >> >       info->cookie =3D 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_m=
mio.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 =3D to_virtio_mmio_device(v=
q->vdev);
>> >> > +     u32 data =3D 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.
>>=20
>> Hm, how noticable is that branch? IOW, is it worth making the code less
>> readable for this?
>
> I'm not sure but these things add up. I'm with Viktor here let's just
> avoid the branch and not worry about whether it's important or not.
> So let's use the same thing here then? And we can use a subfunction
> to avoid code duplication.

Ok, let's do it that way.

>
>> (In any case, all transports probably should use the same method.)