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

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Date:   Wed, 27 Jul 2022 00:50:33 -0700
From:   Si-Wei Liu <si-wei.liu@...cle.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Parav Pandit <parav@...dia.com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "xieyongji@...edance.com" <xieyongji@...edance.com>,
        "gautam.dawar@....com" <gautam.dawar@....com>,
        "Zhu, Lingshan" <lingshan.zhu@...el.com>
Subject: Re: [PATCH V3 5/6] vDPA: answer num of queue pairs = 1 to userspace
 when VIRTIO_NET_F_MQ == 0



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@...el.com>
>>> Sent: Tuesday, July 26, 2022 10:53 PM
>>>
>>> On 7/27/2022 10:17 AM, Parav Pandit wrote:
>>>>> From: Zhu, Lingshan <lingshan.zhu@...el.com>
>>>>> Sent: Tuesday, July 26, 2022 10:15 PM
>>>>>
>>>>> On 7/26/2022 11:56 PM, Parav Pandit wrote:
>>>>>>> From: Zhu, Lingshan <lingshan.zhu@...el.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