Virtualization - Apr 2021 - [PATCH v6 09/10] vduse: Introduce VDUSE - vDPA Device in Userspace

? 2021/4/13 ??12:28, Yongji Xie ??:
> On Tue, Apr 13, 2021 at 11:35 AM Jason Wang <jasowang at redhat.com>
wrote:
>>
>> ? 2021/4/12 ??5:59, Yongji Xie ??:
>>> On Mon, Apr 12, 2021 at 5:37 PM Jason Wang <jasowang at
redhat.com> wrote:
>>>> ? 2021/4/12 ??4:02, Yongji Xie ??:
>>>>> On Mon, Apr 12, 2021 at 3:16 PM Jason Wang <jasowang at
redhat.com> wrote:
>>>>>> ? 2021/4/9 ??4:02, Yongji Xie ??:
>>>>>>>>>>> +};
>>>>>>>>>>> +
>>>>>>>>>>> +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


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