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Subject: Re: [PATCH v8 09/10] vduse: Introduce VDUSE - vDPA Device in
 Userspace
To:     Yongji Xie <xieyongji@bytedance.com>
Cc:     "Michael S. Tsirkin" <mst@redhat.com>,
        Stefan Hajnoczi <stefanha@redhat.com>,
        Stefano Garzarella <sgarzare@redhat.com>,
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From:   Jason Wang <jasowang@redhat.com>
Message-ID: <e82766ff-dc6b-2cbb-3504-0ef618d538e2@redhat.com>
Date:   Tue, 29 Jun 2021 11:29:25 +0800
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在 2021/6/29 上午10:26, Yongji Xie 写道:
> On Mon, Jun 28, 2021 at 12:40 PM Jason Wang <jasowang@redhat.com> wrote:
>>
>> 在 2021/6/25 下午12:19, Yongji Xie 写道:
>>>> 2b) for set_status(): simply relay the message to userspace, reply is no
>>>> needed. Userspace will use a command to update the status when the
>>>> datapath is stop. The the status could be fetched via get_stats().
>>>>
>>>> 2b looks more spec complaint.
>>>>
>>> Looks good to me. And I think we can use the reply of the message to
>>> update the status instead of introducing a new command.
>>>
>> Just notice this part in virtio_finalize_features():
>>
>>           virtio_add_status(dev, VIRTIO_CONFIG_S_FEATURES_OK);
>>           status = dev->config->get_status(dev);
>>           if (!(status & VIRTIO_CONFIG_S_FEATURES_OK)) {
>>
>> So we no reply doesn't work for FEATURES_OK.
>>
>> So my understanding is:
>>
>> 1) We must not use noreply for set_status()
>> 2) We can use noreply for get_status(), but it requires a new ioctl to
>> update the status.
>>
>> So it looks to me we need synchronize for both get_status() and
>> set_status().
>>
> We should not send messages to userspace in the FEATURES_OK case. So
> the synchronization is not necessary.


As discussed previously, there could be a device that mandates some 
features (VIRTIO_F_RING_PACKED). So it can choose to not accept 
FEATURES_OK is packed virtqueue is not negotiated.

In this case we need to relay the message to userspace.

Thanks


>
> Thanks,
> Yongji
>