linux-kernel - Re: [PATCH 2/4] RDMA/core: Resolve MAC of next-hop device without ARP support

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Message-ID: <20250915163002.GJ882933@ziepe.ca>
Date: Mon, 15 Sep 2025 13:30:02 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Parav Pandit <parav@...dia.com>
Cc: Leon Romanovsky <leon@...nel.org>, Edward Srouji <edwards@...dia.com>,
	"linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Cosmin Ratiu <cratiu@...dia.com>,
	Vlad Dumitrescu <vdumitrescu@...dia.com>,
	"kuba@...nel.org" <kuba@...nel.org>,
	Tariq Toukan <tariqt@...dia.com>, Mark Bloch <mbloch@...dia.com>,
	Gal Pressman <gal@...dia.com>
Subject: Re: [PATCH 2/4] RDMA/core: Resolve MAC of next-hop device without
 ARP support

On Wed, Sep 10, 2025 at 10:55:36AM +0000, Parav Pandit wrote:
> > > This leads to incorrect behavior and may result in packet transmission
> > failures.
> > >
> > > Fix this by deferring MAC resolution to the IP stack via neighbour
> > > lookup, allowing proper resolution or error reporting as appropriate.
> > 
> > What is the difference here? For IPv4, neighbour lookup is ARP, no?
> It is but it is not the only way. A device may not do ARP by itself but it relies on the rest of the stack like vrf or ip vlan mode to resolve.
> A user may also set manual entry without explicit ARP.

I think it was just a mistake to use NOARP this way in RDMA, I looked
in the git history and there was no justification. That or it was
right in the 2.x days and netdev moved on to the current schem.

I expect to just call the neighbor functions and if they can't work
for some reason they should fail?

Jason