Re: [RFC 05/10] vhost: Add vhost_dev_from_virtio

[Eugenio Perez Martin]

On Tue, Feb 2, 2021 at 4:31 AM Jason Wang <jasowang@redhat.com> wrote:
>
>
> On 2021/2/1 下午4:28, Eugenio Perez Martin wrote:
> > On Mon, Feb 1, 2021 at 7:13 AM Jason Wang <jasowang@redhat.com> wrote:
> >>
> >> On 2021/1/30 上午4:54, Eugenio Pérez wrote:
> >>> Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> >>> ---
> >>>    include/hw/virtio/vhost.h |  1 +
> >>>    hw/virtio/vhost.c         | 17 +++++++++++++++++
> >>>    2 files changed, 18 insertions(+)
> >>>
> >>> diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
> >>> index 4a8bc75415..fca076e3f0 100644
> >>> --- a/include/hw/virtio/vhost.h
> >>> +++ b/include/hw/virtio/vhost.h
> >>> @@ -123,6 +123,7 @@ uint64_t vhost_get_features(struct vhost_dev *hdev, 
> >>> const int *feature_bits,
> >>>    void vhost_ack_features(struct vhost_dev *hdev, const int 
> >>> *feature_bits,
> >>>                            uint64_t features);
> >>>    bool vhost_has_free_slot(void);
> >>> +struct vhost_dev *vhost_dev_from_virtio(const VirtIODevice *vdev);
> >>>
> >>>    int vhost_net_set_backend(struct vhost_dev *hdev,
> >>>                              struct vhost_vring_file *file);
> >>> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> >>> index 28c7d78172..8683d507f5 100644
> >>> --- a/hw/virtio/vhost.c
> >>> +++ b/hw/virtio/vhost.c
> >>> @@ -61,6 +61,23 @@ bool vhost_has_free_slot(void)
> >>>        return slots_limit > used_memslots;
> >>>    }
> >>>
> >>> +/*
> >>> + * Get the vhost device associated to a VirtIO device.
> >>> + */
> >>> +struct vhost_dev *vhost_dev_from_virtio(const VirtIODevice *vdev)
> >>> +{
> >>> +    struct vhost_dev *hdev;
> >>> +
> >>> +    QLIST_FOREACH(hdev, &vhost_devices, entry) {
> >>> +        if (hdev->vdev == vdev) {
> >>> +            return hdev;
> >>> +        }
> >>> +    }
> >>> +
> >>> +    assert(hdev);
> >>> +    return NULL;
> >>> +}
> >>
> >> I'm not sure this can work in the case of multiqueue. E.g vhost-net
> >> multiqueue is a N:1 mapping between vhost devics and virtio devices.
> >>
> >> Thanks
> >>
> > Right. We could add an "vdev vq index" parameter to the function in
> > this case, but I guess the most reliable way to do this is to add a
> > vhost_opaque value to VirtQueue, as Stefan proposed in previous RFC.
>
>
> So the question still, it looks like it's easier to hide the shadow
> virtqueue stuffs at vhost layer instead of expose them to virtio layer:
>
> 1) vhost protocol is stable ABI
> 2) no need to deal with virtio stuffs which is more complex than vhost
>
> Or are there any advantages if we do it at virtio layer?
>

As far as I can tell, we will need the virtio layer the moment we
start copying/translating buffers.

In this series, the virtio dependency can be reduced if qemu does not
check the used ring _F_NO_NOTIFY flag before writing to irqfd. It
would enable packed queues and IOMMU immediately, and I think the cost
should not be so high. In the previous RFC this check was deleted
later anyway, so I think it was a bad idea to include it from the start.





> Thanks
>
>
> >
> > I need to take this into account in qmp_x_vhost_enable_shadow_vq too.
> >
> >>> +
> >>>    static void vhost_dev_sync_region(struct vhost_dev *dev,
> >>>                                      MemoryRegionSection *section,
> >>>                                      uint64_t mfirst, uint64_t mlast,
>