Re: [PATCH V3 5/6] vDPA: answer num of queue pairs = 1 to userspace when VIRTIO_NET_F_MQ == 0

["Zhu, Lingshan" <lingshan.zhu@intel.com>]

On 7/29/2022 5:54 AM, Si-Wei Liu wrote:
>
>
> On 7/27/2022 7:44 PM, Zhu, Lingshan wrote:
>>
>>
>> On 7/28/2022 9:41 AM, Si-Wei Liu wrote:
>>>
>>>
>>> On 7/27/2022 4:54 AM, Zhu, Lingshan wrote:
>>>>
>>>>
>>>> On 7/27/2022 6:09 PM, Si-Wei Liu wrote:
>>>>>
>>>>>
>>>>> On 7/27/2022 2:01 AM, Michael S. Tsirkin wrote:
>>>>>> On Wed, Jul 27, 2022 at 12:50:33AM -0700, Si-Wei Liu wrote:
>>>>>>>
>>>>>>> On 7/26/2022 11:01 PM, Michael S. Tsirkin wrote:
>>>>>>>> On Wed, Jul 27, 2022 at 03:47:35AM +0000, Parav Pandit wrote:
>>>>>>>>>> From: Zhu, Lingshan <lingshan.zhu@intel.com>
>>>>>>>>>> Sent: Tuesday, July 26, 2022 10:53 PM
>>>>>>>>>>
>>>>>>>>>> On 7/27/2022 10:17 AM, Parav Pandit wrote:
>>>>>>>>>>>> From: Zhu, Lingshan <lingshan.zhu@intel.com>
>>>>>>>>>>>> Sent: Tuesday, July 26, 2022 10:15 PM
>>>>>>>>>>>>
>>>>>>>>>>>> On 7/26/2022 11:56 PM, Parav Pandit wrote:
>>>>>>>>>>>>>> From: Zhu, Lingshan <lingshan.zhu@intel.com>
>>>>>>>>>>>>>> Sent: Tuesday, July 12, 2022 11:46 PM
>>>>>>>>>>>>>>> When the user space which invokes netlink commands, 
>>>>>>>>>>>>>>> detects that
>>>>>>>>>>>> _MQ
>>>>>>>>>>>>>> is not supported, hence it takes max_queue_pair = 1 by 
>>>>>>>>>>>>>> itself.
>>>>>>>>>>>>>> I think the kernel module have all necessary information 
>>>>>>>>>>>>>> and it is
>>>>>>>>>>>>>> the only one which have precise information of a device, 
>>>>>>>>>>>>>> so it
>>>>>>>>>>>>>> should answer precisely than let the user space guess. 
>>>>>>>>>>>>>> The kernel
>>>>>>>>>>>>>> module should be reliable than stay silent, leave the 
>>>>>>>>>>>>>> question to
>>>>>>>>>>>>>> the user space
>>>>>>>>>>>> tool.
>>>>>>>>>>>>> Kernel is reliable. It doesn’t expose a config space field 
>>>>>>>>>>>>> if the
>>>>>>>>>>>>> field doesn’t
>>>>>>>>>>>> exist regardless of field should have default or no default.
>>>>>>>>>>>> so when you know it is one queue pair, you should answer 
>>>>>>>>>>>> one, not try
>>>>>>>>>>>> to guess.
>>>>>>>>>>>>> User space should not guess either. User space gets to see 
>>>>>>>>>>>>> if _MQ
>>>>>>>>>>>> present/not present. If _MQ present than get reliable data 
>>>>>>>>>>>> from kernel.
>>>>>>>>>>>>> If _MQ not present, it means this device has one VQ pair.
>>>>>>>>>>>> it is still a guess, right? And all user space tools 
>>>>>>>>>>>> implemented this
>>>>>>>>>>>> feature need to guess
>>>>>>>>>>> No. it is not a guess.
>>>>>>>>>>> It is explicitly checking the _MQ feature and deriving the 
>>>>>>>>>>> value.
>>>>>>>>>>> The code you proposed will be present in the user space.
>>>>>>>>>>> It will be uniform for _MQ and 10 other features that are 
>>>>>>>>>>> present now and
>>>>>>>>>> in the future.
>>>>>>>>>> MQ and other features like RSS are different. If there is no 
>>>>>>>>>> _RSS_XX, there
>>>>>>>>>> are no attributes like max_rss_key_size, and there is not a 
>>>>>>>>>> default value.
>>>>>>>>>> But for MQ, we know it has to be 1 wihtout _MQ.
>>>>>>>>> "we" = user space.
>>>>>>>>> To keep the consistency among all the config space fields.
>>>>>>>> Actually I looked and the code some more and I'm puzzled:
>>>>>>>>
>>>>>>>>
>>>>>>>>     struct virtio_net_config config = {};
>>>>>>>>     u64 features;
>>>>>>>>     u16 val_u16;
>>>>>>>>
>>>>>>>>     vdpa_get_config_unlocked(vdev, 0, &config, sizeof(config));
>>>>>>>>
>>>>>>>>     if (nla_put(msg, VDPA_ATTR_DEV_NET_CFG_MACADDR, 
>>>>>>>> sizeof(config.mac),
>>>>>>>>             config.mac))
>>>>>>>>         return -EMSGSIZE;
>>>>>>>>
>>>>>>>>
>>>>>>>> Mac returned even without VIRTIO_NET_F_MAC
>>>>>>>>
>>>>>>>>
>>>>>>>>     val_u16 = le16_to_cpu(config.status);
>>>>>>>>     if (nla_put_u16(msg, VDPA_ATTR_DEV_NET_STATUS, val_u16))
>>>>>>>>         return -EMSGSIZE;
>>>>>>>>
>>>>>>>>
>>>>>>>> status returned even without VIRTIO_NET_F_STATUS
>>>>>>>>
>>>>>>>>     val_u16 = le16_to_cpu(config.mtu);
>>>>>>>>     if (nla_put_u16(msg, VDPA_ATTR_DEV_NET_CFG_MTU, val_u16))
>>>>>>>>         return -EMSGSIZE;
>>>>>>>>
>>>>>>>>
>>>>>>>> MTU returned even without VIRTIO_NET_F_MTU
>>>>>>>>
>>>>>>>>
>>>>>>>> What's going on here?
>>>>>>>>
>>>>>>>>
>>>>>>> I guess this is spec thing (historical debt), I vaguely recall 
>>>>>>> these fields
>>>>>>> are always present in config space regardless the existence of 
>>>>>>> corresponding
>>>>>>> feature bit.
>>>>>>>
>>>>>>> -Siwei
>>>>>> Nope:
>>>>>>
>>>>>> 2.5.1  Driver Requirements: Device Configuration Space
>>>>>>
>>>>>> ...
>>>>>>
>>>>>> For optional configuration space fields, the driver MUST check 
>>>>>> that the corresponding feature is offered
>>>>>> before accessing that part of the configuration space.
>>>>> Well, this is driver side of requirement. As this interface is for 
>>>>> host admin tool to query or configure vdpa device, we don't have 
>>>>> to wait until feature negotiation is done on guest driver to 
>>>>> extract vdpa attributes/parameters, say if we want to replicate 
>>>>> another vdpa device with the same config on migration destination. 
>>>>> I think what may need to be fix is to move off from using 
>>>>> .vdpa_get_config_unlocked() which depends on feature negotiation. 
>>>>> And/or expose config space register values through another set of 
>>>>> attributes.
>>>> Yes, we don't have to wait for FEATURES_OK. In another patch in 
>>>> this series, I have added a new netlink attr to report the device 
>>>> features, and removed the blocker. So the LM orchestration SW can 
>>>> query the device features of the devices at the destination 
>>>> cluster, and pick a proper one, even mask out some features to meet 
>>>> the LM requirements.
>>> For that end, you'd need to move off from using 
>>> vdpa_get_config_unlocked() which depends on feature negotiation. 
>>> Since this would slightly change the original semantics of each 
>>> field that "vdpa dev config" shows, it probably need another netlink 
>>> command and new uAPI.
>> why not show both device_features and driver_features in "vdpa dev 
>> config show"?
>>
> As I requested in the other email, I'd like to see the proposed 'vdpa 
> dev config ...' example output for various phases in feature 
> negotiation, and the specific use case (motivation) for this proposed 
> output. I am having difficulty to match what you want to do with the 
> patch posted.
The features bits of a device don't depend on the phases, and the 
driver_features only has meaningful values when FEATURES_OK.

Thanks
>
> -Siwei
>
>>>
>>> -Siwei
>>>
>>>
>>>>
>>>> Thanks,
>>>> Zhu Lingshan
>>>>> -Siwei
>>>>>
>>>>>
>>>>>
>>>>>
>>>>
>>>
>>
>