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

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Date:   Fri, 8 Jul 2022 16:13:27 +0000
From:   Parav Pandit <parav@...dia.com>
To:     "Zhu, Lingshan" <lingshan.zhu@...el.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



> 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.

> >
> > 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.
> >