Re: [PATCH v1 0/8] virtio, vhost: Add VIRTIO_F_NOTIFICATION_DATA support

[Eugenio Perez Martin]

On Wed, Mar 6, 2024 at 6:34 AM Jason Wang <jasowang@redhat.com> wrote:
>
> On Tue, Mar 5, 2024 at 3:46 AM Jonah Palmer <jonah.palmer@oracle.com> wrote:
> >
> > The goal of these patches are to add support to a variety of virtio and
> > vhost devices for the VIRTIO_F_NOTIFICATION_DATA transport feature. This
> > feature indicates that a driver will pass extra data (instead of just a
> > virtqueue's index) when notifying the corresponding device.
> >
> > The data passed in by the driver when this feature is enabled varies in
> > format depending on if the device is using a split or packed virtqueue
> > layout:
> >
> >  Split VQ
> >   - Upper 16 bits: shadow_avail_idx
> >   - Lower 16 bits: virtqueue index
> >
> >  Packed VQ
> >   - Upper 16 bits: 1-bit wrap counter & 15-bit shadow_avail_idx
> >   - Lower 16 bits: virtqueue index
> >
> > Also, due to the limitations of ioeventfd not being able to carry the
> > extra provided by the driver, ioeventfd is left disabled for any devices
> > using this feature.
>
> Is there any method to overcome this? This might help for vhost.
>

As a half-baked idea, read(2)ing an eventfd descriptor returns an
8-byte integer already. The returned value of read depends on eventfd
flags, but both have to do with the number of writes of the other end.

My proposal is to replace this value with the last value written by
the guest, so we can extract the virtio notification data from there.
The behavior of read is similar to not-EFD_SEMAPHORE, reading a value
and then blocking if read again without writes. The behavior of KVM
writes is different, as it is not a counter anymore.

Thanks!

> Thanks
>
> >
> > A significant aspect of this effort has been to maintain compatibility
> > across different backends. As such, the feature is offered by backend
> > devices only when supported, with fallback mechanisms where backend
> > support is absent.
> >
> > Jonah Palmer (8):
> >   virtio/virtio-pci: Handle extra notification data
> >   virtio-pci: Lock ioeventfd state with VIRTIO_F_NOTIFICATION_DATA
> >   virtio-mmio: Handle extra notification data
> >   virtio-mmio: Lock ioeventfd state with VIRTIO_F_NOTIFICATION_DATA
> >   virtio-ccw: Handle extra notification data
> >   virtio-ccw: Lock ioeventfd state with VIRTIO_F_NOTIFICATION_DATA
> >   vhost/vhost-user: Add VIRTIO_F_NOTIFICATION_DATA to vhost feature bits
> >   virtio: Add VIRTIO_F_NOTIFICATION_DATA property definition
> >
> >  hw/block/vhost-user-blk.c    |  1 +
> >  hw/net/vhost_net.c           |  2 ++
> >  hw/s390x/s390-virtio-ccw.c   | 16 ++++++++++++----
> >  hw/s390x/virtio-ccw.c        |  6 ++++--
> >  hw/scsi/vhost-scsi.c         |  1 +
> >  hw/scsi/vhost-user-scsi.c    |  1 +
> >  hw/virtio/vhost-user-fs.c    |  2 +-
> >  hw/virtio/vhost-user-vsock.c |  1 +
> >  hw/virtio/virtio-mmio.c      | 15 +++++++++++----
> >  hw/virtio/virtio-pci.c       | 16 +++++++++++-----
> >  hw/virtio/virtio.c           | 18 ++++++++++++++++++
> >  include/hw/virtio/virtio.h   |  5 ++++-
> >  net/vhost-vdpa.c             |  1 +
> >  13 files changed, 68 insertions(+), 17 deletions(-)
> >
> > --
> > 2.39.3
> >
>