netdev - Re: [PATCH v6 09/10] vduse: Introduce VDUSE

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Date:   Tue, 13 Apr 2021 11:35:00 +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>, 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>,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        kvm@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v6 09/10] vduse: Introduce VDUSE - vDPA Device in
 Userspace


在 2021/4/12 下午5:59, Yongji Xie 写道:
> On Mon, Apr 12, 2021 at 5:37 PM Jason Wang <jasowang@...hat.com> wrote:
>>
>> 在 2021/4/12 下午4:02, Yongji Xie 写道:
>>> On Mon, Apr 12, 2021 at 3:16 PM Jason Wang <jasowang@...hat.com> wrote:
>>>> 在 2021/4/9 下午4:02, Yongji Xie 写道:
>>>>>>>>> +};
>>>>>>>>> +
>>>>>>>>> +struct vduse_dev_config_data {
>>>>>>>>> +     __u32 offset; /* offset from the beginning of config space */
>>>>>>>>> +     __u32 len; /* the length to read/write */
>>>>>>>>> +     __u8 data[VDUSE_CONFIG_DATA_LEN]; /* data buffer used to read/write */
>>>>>>>> Note that since VDUSE_CONFIG_DATA_LEN is part of uAPI it means we can
>>>>>>>> not change it in the future.
>>>>>>>>
>>>>>>>> So this might suffcient for future features or all type of virtio devices.
>>>>>>>>
>>>>>>> Do you mean 256 is no enough here？
>>>>>> Yes.
>>>>>>
>>>>> But this request will be submitted multiple times if config lengh is
>>>>> larger than 256. So do you think whether we need to extent the size to
>>>>> 512 or larger?
>>>> So I think you'd better either:
>>>>
>>>> 1) document the limitation (256) in somewhere, (better both uapi and doc)
>>>>
>>> But the VDUSE_CONFIG_DATA_LEN doesn't mean the limitation of
>>> configuration space. It only means the maximum size of one data
>>> transfer for configuration space. Do you mean document this?
>>
>> Yes, and another thing is that since you're using
>> data[VDUSE_CONFIG_DATA_LEN] in the uapi, it implies the length is always
>> 256 which seems not good and not what the code is wrote.
>>
> How about renaming VDUSE_CONFIG_DATA_LEN to VDUSE_MAX_TRANSFER_LEN?
>
> Thanks,
> Yongji


So a question is the reason to have a limitation of this in the uAPI? 
Note that in vhost-vdpa we don't have such:

struct vhost_vdpa_config {
         __u32 off;
         __u32 len;
         __u8 buf[0];
};

Thanks


>