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

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Date:   Tue, 5 Jul 2022 15:59:23 +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 7/4/2022 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?

IMHO, I don't see any advantages of re-using this attr.

Thanks,
Zhu Lingshan