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

[Eugenio Perez Martin]

On Thu, Feb 4, 2021 at 4:14 AM Jason Wang <jasowang@redhat.com> wrote:
>
>
> On 2021/2/2 下午6:17, Eugenio Perez Martin wrote:
> > 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.
>
>
> I am not sure I understand here. For vhost, we can still do anything we
> want, e.g accessing guest memory etc. Any blocker that prevent us from
> copying/translating buffers? (Note that qemu will propagate memory
> mappings to vhost).
>

There is nothing that forbids us to access directly, but if we don't
reuse the virtio layer functionality we would have to duplicate every
access function. "Need" was a too strong word maybe :).

In other words: for the shadow vq vring exposed for the device, qemu
treats it as a driver, and this functionality needs to be added to
qemu. But for accessing the guest's one do not reuse virtio.c would be
a bad idea in my opinion.

> Thanks
>
>
> >
> >
> >
> >
> >
> >> 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,
> >
>