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

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Date:   Wed, 14 Apr 2021 16:18:11 +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 2021/4/13 下午12:28, Yongji Xie 写道:
> On Tue, Apr 13, 2021 at 11:35 AM Jason Wang <jasowang@...hat.com> wrote:
>>
>> 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];
>> };
>>
> If so, we need to call read()/write() multiple times each time
> receiving/sending one request or response in userspace and kernel. For
> example,
>
> 1. read and check request/response type
> 2. read and check config length if type is VDUSE_SET_CONFIG or VDUSE_GET_CONFIG
> 3. read the payload
>
> Not sure if it's worth it.
>
> Thanks,
> Yongji


Right, I see.

So I'm fine with current approach.

Thanks



>