Re: [PATCH net-next v8 2/5] net: stmmac: drop split frames on AF_XDP zero-copy RX

[Lorenzo Bianconi <lorenzo.bianconi@oss.qualcomm.com>]

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> Hi Lorenzo
> 
> On 18.10.2026 19:07, Lorenzo Bianconi wrote:
> > stmmac_rx_zc() assumes a 1:1 mapping between a received frame and an
> > XSK pool buffer and drops any frame that spans more than one RX
> > descriptor:
> > 
> >         /* XSK pool expects RX frame 1:1 mapped to XSK buffer */
> >         if (likely(status & rx_not_ls)) {
> >                 xsk_buff_free(buf->xdp);
> >                 buf->xdp = NULL;
> >                 dirty++;
> >                 count++;
> >                 goto read_again;
> >         }
> > 
> > The non-last descriptors are freed and dropped, but the frame is not
> > marked as errored. When the poll reaches the last descriptor of the
> > same frame, error is still 0, so the fragment is handled as a complete
> > frame: it is passed to the XDP program, copied into an skb on XDP_PASS,
> > or converted for XDP_TX. The tail starts in the middle of the original
> > payload at a predictable offset and, parsed as a fresh frame (len == 0),
> > stmmac_rx_buf1_len() reports the full dma_buf_sz length for it even
> > though the DMA engine only wrote the tail remainder into that buffer, so
> > the reads extend past the received bytes and can reach beyond the end of
> > the XSK pool buffer. GMAC4 always enables jumbo frames (GMAC_CONFIG_JE
> > in GMAC_CORE_INIT), so an on-link peer can trigger such a split even
> > when the interface MTU is left at 1500.
> > 
> > Set error when the 1:1 mapping check drops a descriptor, so the
> > remaining descriptors of the frame - including the last one - are
> > drained and released with xsk_buff_free() instead of being processed as
> > a new frame. Account every freed buffer with dirty and count the
> > dropped frame once in the poll budget / rx_pkt_n. The direct 1:1
> > mapping drop also increments rx_dropped.
> > 
> > Normalize the legacy normal and enhanced descriptor callbacks to report
> > rx_not_ls together with discard_frame for non-last descriptors. This
> > allows both stmmac_rx_zc() and stmmac_rx() to drain split frames on
> > dwmac100 and dwmac1000 instead of reparsing the final fragment as a new
> > packet.
> > 
> > Fixes: bba2556efad6 ("net: stmmac: Enable RX via AF_XDP zero-copy")
> > Reviewed-by: Nicolai Buchwitz <nb@tipi-net.de>
> 
> This grew quite a bit since v6 (legacy desc changes, stmmac_rx() etc),
> so my R-b should have been dropped. I had another look and two small
> notes below.

Hi Nicolai,

ack, right.

> 
> > Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@oss.qualcomm.com>
> > ---
> 
> > [...]
> 
> > --- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> > +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> > @@ -5695,18 +5695,21 @@ static int stmmac_rx_zc(struct stmmac_priv
> > *priv, int limit, u32 queue)
> >  		if (priv->extend_desc)
> >  			stmmac_rx_extended_status(priv, &priv->xstats,
> >  						  rx_q->dma_erx + entry);
> > -		if (unlikely(status == discard_frame)) {
> > -			xsk_buff_free(buf->xdp);
> > -			buf->xdp = NULL;
> > -			dirty++;
> > +
> > +		if (unlikely(status & discard_frame)) {
> >  			error = 1;
> >  			if (!priv->hwts_rx_en)
> >  				rx_errors++;
> >  		}
> 
> There's no !error check here, so on the legacy descriptors a split
> frame now counts once per descriptor in rx_errors. stmmac_rx() in 3/5
> counts it once. Could you add the !error check here too?

ack, I agree we should align stmmac_rx() and stmmac_rx_zc(). In order to avoid
reposting the full series for this change, do you agree to address it as a
follow-up patch?

Regards,
Lorenzo

> 
> > [...]
> 
> > @@ -5883,7 +5887,8 @@ static int stmmac_rx(struct stmmac_priv *priv, int
> > limit, u32 queue)
> > 
> >  		if (priv->extend_desc)
> >  			stmmac_rx_extended_status(priv, &priv->xstats, rx_q->dma_erx +
> > entry);
> > -		if (unlikely(status == discard_frame)) {
> > +
> > +		if (unlikely(status & discard_frame)) {
> 
> AFAIU this also changes GMAC4: a frame with an error plus an IP header
> error returns discard_frame | csum_none, which the old "==" let
> through. Dropping it is right, but maybe worth a line in the commit
> message?
> 
> > [...]
> 
> With the !error check added:
> 
> Reviewed-by: Nicolai Buchwitz <nb@tipi-net.de>
> 
> Thanks,
> Nicolai

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