[PATCH] xfrm: Add pre-encap fragmentation for packet offload

[Ilia Lin <ilia.lin@kernel.org>]

On Mon, Nov 25, 2024 at 9:43 PM Leon Romanovsky <leon@kernel.org> wrote:
>
> On Mon, Nov 25, 2024 at 11:26:14AM +0200, Ilia Lin wrote:
> > On Sun, Nov 24, 2024 at 2:04 PM Leon Romanovsky <leon@kernel.org> wrote:
> > >
> > > On Sun, Nov 24, 2024 at 11:35:31AM +0200, Ilia Lin wrote:
> > > > In packet offload mode the raw packets will be sent to the NiC,
> > > > and will not return to the Network Stack. In event of crossing
> > > > the MTU size after the encapsulation, the NiC HW may not be
> > > > able to fragment the final packet.
> > >
> > > Yes, HW doesn't know how to handle these packets.
> > >
> > > > Adding mandatory pre-encapsulation fragmentation for both
> > > > IPv4 and IPv6, if tunnel mode with packet offload is configured
> > > > on the state.
> > >
> > > I was under impression is that xfrm_dev_offload_ok() is responsible to
> > > prevent fragmentation.
> > > https://elixir.bootlin.com/linux/v6.12/source/net/xfrm/xfrm_device.c#L410
> >
> > With my change we can both support inner fragmentation or prevent it,
> > depending on the network device driver implementation.
>
> The thing is that fragmentation isn't desirable thing. Why didn't PMTU
> take into account headers so we can rely on existing code and do not add
> extra logic for packet offload?

I agree that PMTU is a preferred option, but the packets may be routed from
a host behind the VPN, which is unaware that it transmits into an IPsec tunnel,
and therefore will not count on the extra headers.

>
> Thanks
>
> >
> > >
> > > Thanks