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 11:56:49 +0000
From:   Parav Pandit <parav@...dia.com>
To:     "Zhu, Lingshan" <lingshan.zhu@...el.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


> 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.

> 
> 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.