netdev - Re: [PATCH v8 09/10] vduse: Introduce VDUSE

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Date:   Fri, 25 Jun 2021 11:08:42 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Yongji Xie <xieyongji@...edance.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        Stefano Garzarella <sgarzare@...hat.com>,
        Parav Pandit <parav@...dia.com>,
        Christoph Hellwig <hch@...radead.org>,
        Christian Brauner <christian.brauner@...onical.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Jens Axboe <axboe@...nel.dk>, bcrl@...ck.org,
        Jonathan Corbet <corbet@....net>,
        Mika Penttilä <mika.penttila@...tfour.com>,
        Dan Carpenter <dan.carpenter@...cle.com>, joro@...tes.org,
        Greg KH <gregkh@...uxfoundation.org>, songmuchun@...edance.com,
        virtualization <virtualization@...ts.linux-foundation.org>,
        netdev@...r.kernel.org, kvm <kvm@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v8 09/10] vduse: Introduce VDUSE - vDPA Device in
 Userspace


在 2021/6/24 下午5:16, Yongji Xie 写道:
> On Thu, Jun 24, 2021 at 4:14 PM Jason Wang <jasowang@...hat.com> wrote:
>>
>> 在 2021/6/24 下午12:46, Yongji Xie 写道:
>>>> So we need to deal with both FEATURES_OK and reset, but probably not
>>>> DRIVER_OK.
>>>>
>>> OK, I see. Thanks for the explanation. One more question is how about
>>> clearing the corresponding status bit in get_status() rather than
>>> making set_status() fail. Since the spec recommends this way for
>>> validation which is done in virtio_dev_remove() and
>>> virtio_finalize_features().
>>>
>>> Thanks,
>>> Yongji
>>>
>> I think you can. Or it would be even better that we just don't set the
>> bit during set_status().
>>
> Yes, that's what I mean.
>
>> I just realize that in vdpa_reset() we had:
>>
>> static inline void vdpa_reset(struct vdpa_device *vdev)
>> {
>>           const struct vdpa_config_ops *ops = vdev->config;
>>
>>           vdev->features_valid = false;
>>           ops->set_status(vdev, 0);
>> }
>>
>> We probably need to add the synchronization here. E.g re-read with a
>> timeout.
>>
> Looks like the timeout is already in set_status().


Do you mean the VDUSE's implementation?


>   Do we really need a
> duplicated one here?


1) this is the timeout at the vDPA layer instead of the VDUSE layer.
2) it really depends on what's the meaning of the timeout for set_status 
of VDUSE.

Do we want:

2a) for set_status(): relay the message to userspace and wait for the 
userspace to quiescence the datapath

or

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.

> And how to handle failure? Adding a return value
> to virtio_config_ops->reset() and passing the error to the upper
> layer?


Something like this.

Thanks


>
> Thanks,
> Yongji
>