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Date:   Wed, 27 Jul 2022 14:25:59 +0800
From:   "Zhu, Lingshan" <>
To:     "Michael S. Tsirkin" <>,
        Parav Pandit <>
Cc:     "" <>,
        "" <>,
        "" <>,
        "" <>
Subject: Re: [PATCH V3 5/6] vDPA: answer num of queue pairs = 1 to userspace
 when VIRTIO_NET_F_MQ == 0

On 7/27/2022 2:01 PM, Michael S. Tsirkin wrote:
> On Wed, Jul 27, 2022 at 03:47:35AM +0000, Parav Pandit wrote:
>>> From: Zhu, Lingshan <>
>>> Sent: Tuesday, July 26, 2022 10:53 PM
>>> On 7/27/2022 10:17 AM, Parav Pandit wrote:
>>>>> From: Zhu, Lingshan <>
>>>>> Sent: Tuesday, July 26, 2022 10:15 PM
>>>>> On 7/26/2022 11:56 PM, Parav Pandit wrote:
>>>>>>> From: Zhu, Lingshan <>
>>>>>>> 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:
I can submit a fix in my next version patch for these issue.
> 	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
if no VIRTIO_NET_F_MAC, we should not nla_put 
VDPA_ATTR_DEV_NET_CFG_MAC_ADDR, the spec says the driver should generate 
a random 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
if no VIRTIO_NET_F_STATUS, we should not nla_put 
VDPA_ATTR_DEV_NET_STATUS, the spec says the driver should assume the 
link is active.
> 	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
same as above, the spec says config.mtu depends on VIRTIO_NET_F_MTU, so 
without this feature bit, we should not return MTU to the userspace.

Does these fix look good to you?

And I think we may need your adjudication for the two issues:
(1) Shall we answer max_vq_paris = 1 when _MQ not exist, I know you have 
agreed on this in a previous thread, its nice to clarify
(2) I think we should not re-use the netlink attr to report feature bits 
of both the management device and the vDPA device,
this can lead to a new race condition, there are no locks(especially 
distributed locks for kernel_space and user_space) in the nla_put
functions. Re-using the attr is some kind of breaking the netlink 
lockless design.

Zhu Lingshan
> What's going on here?

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