Re: [PATCH net] igc: track rx frame boundaries independent of skbs

[Maciej Fijalkowski <maciej.fijalkowski@intel.com>]

On Mon, Oct 26, 2026 at 10:12:51PM +0000, Paul Moses wrote:
> 
> 
> 
> 
> On Monday, October 26th, 2026 at 2:46 PM, Maciej Fijalkowski <maciej.fijalkowski@intel.com> wrote:
> 
> > On Sun, Oct 25, 2026 at 06:52:06AM +0000, Paul Moses wrote:
> > > TSIP remains set on continuation descriptors, but the inline timestamp
> > > header exists only in the first buffer. Stripping it from every
> > > descriptor truncates split frames and can underflow short buffers.
> > >
> > > skb ownership cannot identify frame boundaries when XDP consumes the
> > > first buffer or AF_XDP uses zero-copy.
> > >
> > > Track rx_in_frame per ring and update it as descriptors are consumed.
> > > Strip timestamps only from first buffers, reject unsupported split
> > > frames before XDP and drain continuations. Account discarded
> > > descriptors against NAPI work without counting successful RX.
> > >
> > > Fixes: e1ed4f92a625 ("igc: Refactor Rx timestamp handling")
> > > Fixes: 26575105d6ed ("igc: Add initial XDP support")
> > > Fixes: fc9df2a0b520 ("igc: Enable RX via AF_XDP zero-copy")
> > > Cc: stable@vger.kernel.org
> > > Signed-off-by: Paul Moses <p@1g4.org>
> > > ---
> > > - This supersedes "igc: only strip RX timestamp header from first buffer"
> > >   following feedback from Simon Horman, Maciej Fijalkowski and Sashiko.
> > >
> > > Changes:
> > > - track frame boundaries independently of skb ownership
> > > - guard timestamp stripping and drain unsupported split frames
> > > - cover the AF_XDP zero-copy RX path
> > > ---
> > >  drivers/net/ethernet/intel/igc/igc.h      |  1 +
> > >  drivers/net/ethernet/intel/igc/igc_main.c | 87 ++++++++++++++++-------
> > >  2 files changed, 64 insertions(+), 24 deletions(-)
> > 
> > Hi Paul,
> > 
> > I think this implementation went off track. We should make sure we *never*
> > enter data path for XDP/AF_XDP multi-buffer scenario until we add real
> > support to it within this driver. Now you sorta handle fragmented packets
> > in data path.
> > 
> > I tried to argue with bot reports that it hallucinated for
> > igc_clean_rx_irq() content (which was concise and to the point) and agreed
> > the AF_XDP should *not* be exposed to any fragmented traffic, which should
> > be addressed on HW RX ring configuration side, not within data path.
> > 
> > Also this should have been a v3 with a link to previous revisions so we
> > could keep track what has been changing between versions.
> > 
> > Here's something that would address ZC fragmentation issue. Could you give
> > it a spin on your side as I don't currently have igc in my setup handy.

[...]

> 
> Looks good to me, passes basic testing, and I would rather you/intel do this patch
> 
> For posterity, I did email Tony with handoff notes stating I'd rather not do the 
> xdp/xsk/af_xdp/fpe side of things, but hadn't heard back so I kept going. 

I think this was within the thread where AI hallucinated and I stepped in.
If you are not fed up yet you can take your v2 and the patch I added here
and send this as a change set. From my POV it would be better as you have
the HW to test this against and ironically my igc card went into
/dev/null's lab.

>