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Date: Thu, 30 Oct 2025 08:55:35 -0700
From: Jakub Kicinski <kuba@kernel.org>
To: Alexander Lobakin <aleksander.lobakin@intel.com>
Cc: Paolo Abeni <pabeni@redhat.com>, Jason Xing <kerneljasonxing@gmail.com>,
 <davem@davemloft.net>, <edumazet@google.com>, <bjorn@kernel.org>,
 <magnus.karlsson@intel.com>, <maciej.fijalkowski@intel.com>,
 <jonathan.lemon@gmail.com>, <sdf@fomichev.me>, <ast@kernel.org>,
 <daniel@iogearbox.net>, <hawk@kernel.org>, <john.fastabend@gmail.com>,
 <joe@dama.to>, <willemdebruijn.kernel@gmail.com>, <bpf@vger.kernel.org>,
 <netdev@vger.kernel.org>, Jason Xing <kernelxing@tencent.com>
Subject: Re: [PATCH net-next v2] xsk: add indirect call for xsk_destruct_skb
Message-ID: <20251030085535.4f658dd8@kernel.org>
In-Reply-To: <e290a675-fc1e-4edf-833c-aa82af073d30@intel.com>
References: <20251026145824.81675-1-kerneljasonxing@gmail.com>
	<54d1ac44-8e53-4056-8061-0c620d9ec4bf@redhat.com>
	<e290a675-fc1e-4edf-833c-aa82af073d30@intel.com>
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On Thu, 30 Oct 2025 11:59:58 +0100 Alexander Lobakin wrote:
> >> managed to see a huge improvement[1], the same situation can also be
> >> applied in xsk scenario.
> >>
> >> This patch adds an indirect call for xsk and helps current copy mode
> >> improve the performance by around 1% stably which was observed with
> >> IXGBE at 10Gb/sec loaded.   
> >
> > If I follow the conversation correctly, Jakub's concern is mostly about
> > this change affecting only the copy mode.
> >
> > Out of sheer ignorance on my side is not clear how frequent that
> > scenario is. AFAICS, applications could always do zero-copy with proper
> > setup, am I correct?!?  
>
> It is correct only when the target driver implements zero-copy
> driver-side XSk. While it's true for modern Ethernet drivers for real
> NICs, "virtual" drivers like virtio-net, veth etc. usually don't have it.
> It's not as common usecase as using XSk on real NICs, but still valid
> and widely used.

To be clear my main concern is that the XDP<>skb conversions are 
an endless source of bugs and complexity. We have one fix for XDP->skb
on the list from Maciej and another for AF_XDP from Fernando which
tried to create an XDP skb_ext. We are digging a deeper and deeper
hole with all this fallback stuff, and it will affect performance
of both normal skb and XDP paths. Optimizing AF_XDP fallback is
shortsighted.