Message-ID: <cbbb0341-5383-0e64-1ab8-52289869a233@intel.com>
Date:   Mon, 11 Jul 2022 10:18:53 +0800
From:   "Zhu, Lingshan" <lingshan.zhu@...el.com>
To:     Parav Pandit <parav@...dia.com>,
        "jasowang@...hat.com" <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 7/9/2022 12:13 AM, Parav Pandit wrote:
>
>> From: Zhu, Lingshan <lingshan.zhu@...el.com>
>> Sent: Friday, July 8, 2022 2:16 AM
>>
>> On 7/2/2022 6:02 AM, Parav Pandit wrote:
>>>> 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.
>> will fix
>>> I reviewed the patches again.
>>>
>>> However, this is not the fix.
>>> A fix cannot add a new UAPI.
>> I think we have discussed this, on why we can not re-name the existing
>> wrong named attr, and why we can not re-use the attr.
>> So are you suggesting remove this fixes tag?
>> And why a fix can not add a new uAPI?
> Because a new attribute cannot fix any existing attribute.
>
> What is done in the patch is show current attributes of the vdpa device (which sometimes contains a different value than the mgmt. device).
> So it is a new functionality that cannot have fixes tag.
OK, I get the points now.
>
>>> Code is already considering negotiated driver features to return the device
>> config space.
>>> Hence it is fine.
>> No, the spec says:
>> The device MUST allow reading of any device-specific configuration field
>> before FEATURES_OK is set by the driver.
>>> 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].
>> we have discussed this in another thread, vDPA device feature bits can be
>> different from the management device feature bits.
> Yes.
>>> 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.
>> Yes, rename it will break current uAPI, so I can not rename it.
> I know, which is why this patch needs to do following listed changes described in previous email.
>
>>> 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.
>>> Secondly, you need output example for showing device features in the
>> commit log.
>>> 3rd, please drop the fixes tag as new capability is not a fix.
>>>

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