netdev - Re: [PATCH V3 3/6] vDPA: allow userspace to query features of a vDPA device

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Date:   Wed, 6 Jul 2022 00:56:10 +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 7/5/2022 7:56 PM, Parav Pandit wrote:
>> From: Zhu, Lingshan <lingshan.zhu@...el.com>
>> Sent: Tuesday, July 5, 2022 3:59 AM
>>
>>
>> On 7/4/2022 8:53 PM, Parav Pandit wrote:
>>>> From: Jason Wang <jasowang@...hat.com>
>>>> Sent: Monday, July 4, 2022 12:47 AM
>>>>
>>>>
>>>> 在 2022/7/2 06:02, Parav Pandit 写道:
>>>>>> From: Zhu Lingshan <lingshan.zhu@...el.com>
>>>>>> Sent: Friday, July 1, 2022 9:28 AM
>>>>>>
>>>>>> This commit adds a new vDPA netlink attribution
>>>>>> VDPA_ATTR_VDPA_DEV_SUPPORTED_FEATURES. Userspace can query
>>>> features
>>>>>> of vDPA devices through this new attr.
>>>>>>
>>>>>> Fixes: a64917bc2e9b vdpa: (Provide interface to read driver
>>>>>> feature)
>>>>> Missing the "" in the line.
>>>>> I reviewed the patches again.
>>>>>
>>>>> However, this is not the fix.
>>>>> A fix cannot add a new UAPI.
>>>>>
>>>>> Code is already considering negotiated driver features to return the
>>>>> device
>>>> config space.
>>>>> Hence it is fine.
>>>>>
>>>>> This patch intents to provide device features to user space.
>>>>> First what vdpa device are capable of, are already returned by
>>>>> features
>>>> attribute on the management device.
>>>>> This is done in commit [1].
>>>>>
>>>>> The only reason to have it is, when one management device indicates
>>>>> that
>>>> feature is supported, but device may end up not supporting this
>>>> feature if such feature is shared with other devices on same physical device.
>>>>> For example all VFs may not be symmetric after large number of them
>>>>> are
>>>> in use. In such case features bit of management device can differ
>>>> (more
>>>> features) than the vdpa device of this VF.
>>>>> Hence, showing on the device is useful.
>>>>>
>>>>> As mentioned before in V2, commit [1] has wrongly named the
>>>>> attribute to
>>>> VDPA_ATTR_DEV_SUPPORTED_FEATURES.
>>>>> It should have been,
>>>> VDPA_ATTR_DEV_MGMTDEV_SUPPORTED_FEATURES.
>>>>> Because it is in UAPI, and since we don't want to break compilation
>>>>> of iproute2, It cannot be renamed anymore.
>>>>>
>>>>> Given that, we do not want to start trend of naming device
>>>>> attributes with
>>>> additional _VDPA_ to it as done in this patch.
>>>>> Error in commit [1] was exception.
>>>>>
>>>>> Hence, please reuse VDPA_ATTR_DEV_SUPPORTED_FEATURES to return
>>>> for device features too.
>>>>
>>>>
>>>> This will probably break or confuse the existing userspace?
>>>>
>>> It shouldn't break, because its new attribute on the device.
>>> All attributes are per command, so old one will not be confused either.
>> A netlink attr should has its own and unique purpose, that's why we don't need
>> locks for the attrs, only one consumer and only one producer.
>> I am afraid re-using (for both management device and the vDPA device) the attr
>> VDPA_ATTR_DEV_SUPPORTED_FEATURES would lead to new race condition.
>> E.g., There are possibilities of querying FEATURES of a management device and
>> a vDPA device simultaneously, or can there be a syncing issue in a tick?
> 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.
And re-use a netlink attr is really confusing.
>
>> 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