RE: [PATCH V3 5/6] vDPA: answer num of queue pairs = 1 to userspace when VIRTIO_NET_F_MQ == 0

From: Parav Pandit <parav@nvidia.com>
To: "Zhu, Lingshan" <lingshan.zhu@intel.com>,
	"jasowang@redhat.com" <jasowang@redhat.com>,
	"mst@redhat.com" <mst@redhat.com>
Cc: "virtualization@lists.linux-foundation.org" 
	<virtualization@lists.linux-foundation.org>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	"xieyongji@bytedance.com" <xieyongji@bytedance.com>,
	"gautam.dawar@amd.com" <gautam.dawar@amd.com>
Subject: RE: [PATCH V3 5/6] vDPA: answer num of queue pairs = 1 to userspace when VIRTIO_NET_F_MQ == 0
Date: Wed, 13 Jul 2022 03:06:17 +0000	[thread overview]
Message-ID: <PH0PR12MB5481E65037E0B4F6F583193BDC899@PH0PR12MB5481.namprd12.prod.outlook.com> (raw)
In-Reply-To: <c7c8f49c-484f-f5b3-39e6-0d17f396cca7@intel.com>

> From: Zhu, Lingshan <lingshan.zhu@intel.com>
> Sent: Tuesday, July 12, 2022 11:03 PM
> 
> 
> On 7/13/2022 12:48 AM, Parav Pandit wrote:
> >> From: Zhu, Lingshan <lingshan.zhu@intel.com>
> >> Sent: Sunday, July 10, 2022 10:30 PM
> >>> Showing max_vq_pairs of 1 even when _MQ is not negotiated,
> >>> incorrectly
> >> says that max_vq_pairs is exposed to the guest, but it is not offered.
> >>> So, please fix the iproute2 to not print max_vq_pairs when it is not
> >> returned by the kernel.
> >> iproute2 can report whether there is MQ feature in the device /
> >> driver feature bits.
> >> I think iproute2 only queries the number of max queues here.
> >>
> >> max_vq_pairs shows how many queue pairs there, this attribute's
> >> existence does not depend on MQ, if no MQ, there are still one queue
> >> pair, so just show one.
> > This netlink attribute's existence is depending on the _MQ feature bit
> existence.
> why? If no MQ, then no queues?
> > We can break that and report the value, but if we break that there are
> many other config space bits who doesn’t have good default like
> max_vq_pairs.
> max_vq_paris may not have a default value, but we know if there is no MQ,
> a virtio-net still have one queue pair to be functional.
> > There is ambiguity for user space what to do with it and so in the kernel
> space..
> > Instead of dealing with them differently in kernel, at present we attach
> each netlink attribute to a respective feature bit wherever applicable.
> > And code in kernel and user space is uniform to handle them.
> I get your point, but you see, by "max_vq_pairs", the user space tool is
> asking how many queue pairs there, it is not asking whether the device have
> MQ.
> Even no _MQ, we still need to tell the users that there are one queue pair, or
> it is not a functional virtio-net, we should detect this error earlier in the
> device initialization.
It is not an error. :)

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 it is still uniform, it there is _MQ, we return cfg.max_queue_pair, if no
> _MQ, return 1, still by netlink.
Better to do that in user space because we cannot do same for other config fields.

> 
> Thanks