Re: [PATCH V2 1/1] Docs: ublk: add ublk document

[Ziyang Zhang <ZiyangZhang@linux.alibaba.com>]

On 2022/9/1 11:12, Ming Lei wrote:
> On Thu, Sep 01, 2022 at 11:04:24AM +0800, Ziyang Zhang wrote:
>> On 2022/9/1 10:30, Ming Lei wrote:
>>> +
>>> +- ``UBLK_IO_NEED_GET_DATA``
>>> +
>>> +  ublk server pre-allocates IO buffer for each IO by default. Any new projects
>>> +  should use this buffer to communicate with ublk driver. However, existing
>>> +  projects may break or not able to consume the new buffer interface; that's
>>> +  why this command is added for backwards compatibility so that existing
>>> +  projects can still consume existing buffers.
>>
>> Hi, Ming.
>>
>> Could you please add more information on UBLK_IO_NEED_GET_DATA. stefanha
>> found it hard to understand.
>>
>> Myabe we should write like this:
>>
>> With UBLK_F_NEED_GET_DATA enabled, the WRITE request will be firstly issued to
>> ublksrv without data copy. Then, IO backend receives the request and it can allocate
>> data buffer and embed its addr inside a new ioucmd. After the kernel driver gets the
>> ioucmd, the data copy happens(from biovecs to backend's buffer). Finally,
>> the backend receives the request again with data to be written and it can truly
>> handle the request.
>>
>> UBLK_IO_NEED_GET_DATA add one additional round-trip in ublk_drv and one
>> io_uring_enter() syscall. Any user thinks that it may lower performance
>> should not enable UBLK_F_NEED_GET_DATA. ublk server pre-allocates IO buffer
>> for each IO by default. Any new projects should use this buffer to communicate
>> with ublk driver. However, existing projects may break or not able to consume
>> the new buffer interface; that's why this command is added for backwards
>> compatibility so that existing projects can still consume existing buffers.
> 
> I am fine to add it if V3 is needed. If not, please send a new patch.
> 
> BTW, I guess Jens may consider it for v6.0.

Please add it to V3, Ming.

Thanks,
Zhang