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

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Message-ID: <PH0PR12MB5481167F0B47A83A2E2E86D4DC949@PH0PR12MB5481.namprd12.prod.outlook.com>
Date:   Tue, 26 Jul 2022 20:49:45 +0000
From:   Parav Pandit <parav@...dia.com>
To:     "Michael S. Tsirkin" <mst@...hat.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


> From: Michael S. Tsirkin <mst@...hat.com>
> Sent: Tuesday, July 26, 2022 3:52 PM
> 
> 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.
I read it differently at [1] which checks for the MQ feature bit.

[1] https://elixir.bootlin.com/linux/latest/source/drivers/vdpa/vdpa.c#L825

> 
> --
> MST