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Date:   Wed, 6 Jul 2022 10:25:15 +0800
From:   "Zhu, Lingshan" <lingshan.zhu@...el.com>
To:     Parav Pandit <parav@...dia.com>, Jason Wang <jasowang@...hat.com>,
        "mst@...hat.com" <mst@...hat.com>
Cc:     "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "xieyongji@...edance.com" <xieyongji@...edance.com>,
        "gautam.dawar@....com" <gautam.dawar@....com>
Subject: Re: [PATCH V3 3/6] vDPA: allow userspace to query features of a vDPA
 device



On 7/6/2022 1:01 AM, Parav Pandit wrote:
>> From: Zhu, Lingshan <lingshan.zhu@...el.com>
>> Sent: Tuesday, July 5, 2022 12:56 PM
>>> Both can be queried simultaneously. Each will return their own feature bits
>> using same attribute.
>>> It wont lead to the race.
>> How? It is just a piece of memory, xxxx[attr], do you see locks in
>> nla_put_u64_64bit()? It is a typical race condition, data accessed by multiple
>> producers / consumers.
> No. There is no race condition in here.
> And new attribute enum by no means avoid any race.
>
> Data put using nla_put cannot be accessed until they are transferred.
How this is guaranteed? Do you see errors when calling nla_put_xxx() twice?
>
>> And re-use a netlink attr is really confusing.
> Please put comment for this variable explaining why it is shared for the exception.
>
> Before that lets start, can you share a real world example of when this feature bitmap will have different value than the mgmt. device bitmap value?
For example,
1. When migrate the VM to a node which has a more resourceful device. If 
the source side device does not have MQ, RSS or TSO feature, the vDPA 
device assigned to the VM does not
have MQ, RSS or TSO as well. When migrating to a node which has a device 
with MQ, RSS or TSO, to provide a consistent network device to the 
guest, to be transparent to the guest,
we need to mask out MQ, RSS or TSO in the vDPA device when provisioning. 
This is an example that management device may have different feature 
bits than the vDPA device.

2.SIOV, if a virtio device is capable of managing SIOV devices, and it 
exposes this capability by a feature bit(Like what I am doing in the 
"transport virtqueue"),
we don't want the SIOV ADIs have SIOV features, so the ADIs don't have 
SIOV feature bit.

Thanks
>
>>>> IMHO, I don't see any advantages of re-using this attr.
>>> We don’t want to continue this mess of VDPA_DEV prefix for new
>> attributes due to previous wrong naming.
>> as you point out before, is is a wrong naming, we can't re-nmme it because
>> we don't want to break uAPI, so there needs a new attr, if you don't like the
>> name VDPA_ATTR_VDPA_DEV_SUPPORTED_FEATURES, it is more than
>> welcome to suggest a new one
>>
>> Thanks

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