From: Jan Klemkow <jan@openbsd.org>
Subject: Re: ixl(4): Fix hardmtu
To: Jonathan Matthew <jonathan@d14n.org>
Cc: tech@openbsd.org
Date: Tue, 22 Sep 2026 05:00:39 +0200

On Fri, Aug 21, 2026 at 06:53:53PM +0200, Jan Klemkow wrote:
> On Thu, Jul 30, 2026 at 06:16:40PM +0200, Jan Klemkow wrote:
> > Using ixl(4) with it's hardmtu is broken, because packets with hardmtu
> > were dropped cause they are bigger then rxmax is configured.
> > 
> > The Intel Ethernet Controller X710/XXV710/XL710 Datasheet said:
> > 
> > Table 1-3. Link layer Ethernet port features
> > 
> > 	Maximum Transmit Unit Size (MTU) is 9728 - Ethernet header/CRC =
> > 	9728 - 18 = 9710 bytes (jumbo frames)
> > 	MTU can be further reduced by additional header fields such as
> > 	Virtual Local Area Network (VLAN) tag(s), etc.
> > 
> > Table 8-12. LAN Rx Queue Context in the Private Host Memory
> > 
> > 	The "rxmax" parameter defines the whole packet size starting at
> > 	the L2 header up to including the Ethernet CRC.
> > 
> > The following diff sending and receiving sizes in the ixl(4) code.  The
> > result is a working hardmtu of 9706 and its also working with vlan(4).

Adjust the rxmax part as the spec said:
	The RXMAX must not be set to a larger value than 5 x DBUFF
	(since receive packet must never span on more than 5 buffers).

ok?

bye,
jan

Index: dev/pci/if_ixl.c
===================================================================
RCS file: /cvs/src/sys/dev/pci/if_ixl.c,v
diff -u -p -r1.117 if_ixl.c
--- dev/pci/if_ixl.c	23 Jun 2026 14:40:40 -0000	1.117
+++ dev/pci/if_ixl.c	22 Sep 2026 02:54:00 -0000
@@ -901,7 +901,8 @@ struct ixl_rx_wb_desc_32 {
 #define IXL_TX_QUEUE_ALIGN		128
 #define IXL_RX_QUEUE_ALIGN		128
 
-#define IXL_HARDMTU			9712 /* 9726 - ETHER_HDR_LEN */
+#define IXL_HARDMTU			(9728 - ETHER_HDR_LEN - EVL_ENCAPLEN \
+					    - ETHER_CRC_LEN)
 #define IXL_TSO_SIZE			((255 * 1024) - 1)
 #define IXL_MAX_DMA_SEG_SIZE		((16 * 1024) - 1)
 
@@ -3171,7 +3172,7 @@ ixl_rxr_config(struct ixl_softc *sc, str
 	rxq.crcstrip = 1;
 	rxq.l2tsel = IXL_HMC_RXQ_L2TSEL_1ST_TAG_TO_L2TAG1;
 	rxq.showiv = 0;
-	rxq.rxmax = htole16(IXL_HARDMTU);
+	rxq.rxmax = htole16(MCLBYTES * 5);
 	rxq.tphrdesc_ena = 0;
 	rxq.tphwdesc_ena = 0;
 	rxq.tphdata_ena = 0;