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

[Jason Wang <jasowang@redhat.com>]

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).

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,
>