netdev - Re: [PATCH V3 5/6] vDPA: answer num of queue pairs = 1 to userspace when VIRTIO_NET_F

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Date:   Tue, 26 Jul 2022 15:52:01 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Parav Pandit <parav@...dia.com>
Cc:     "Zhu, Lingshan" <lingshan.zhu@...el.com>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "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 5/6] vDPA: answer num of queue pairs = 1 to userspace
 when VIRTIO_NET_F_MQ == 0

On Tue, Jul 26, 2022 at 03:56:32PM +0000, Parav Pandit wrote:
> 
> > From: Zhu, Lingshan <lingshan.zhu@...el.com>
> > Sent: Tuesday, July 12, 2022 11:46 PM
> > > When the user space which invokes netlink commands, detects that _MQ
> > is not supported, hence it takes max_queue_pair = 1 by itself.
> > I think the kernel module have all necessary information and it is the only
> > one which have precise information of a device, so it should answer precisely
> > than let the user space guess. The kernel module should be reliable than stay
> > silent, leave the question to the user space tool.
> Kernel is reliable. It doesn’t expose a config space field if the field doesn’t exist regardless of field should have default or no default.
> User space should not guess either. User space gets to see if _MQ present/not present. If _MQ present than get reliable data from kernel.
> If _MQ not present, it means this device has one VQ pair.

Yes that's fine. And if we just didn't return anything without MQ that
would be fine.  But IIUC netlink reports the # of pairs regardless, it
just puts 0 there.

-- 
MST