[UNIKRAFT/LWIP PATCH v5 2/5] cc.h: Disable Ethernet frame padding

[Simon Kuenzer <simon.kuenzer@xxxxxxxxx>]

From: Costin Lupu <costin.lupu@xxxxxxxxx>

We set ETH_PAD_SIZE to 0 and remove support from uknetdev.c. This is done
because some uknetdev drivers will not support it (e.g., netfront) and
according to https://lwn.net/Articles/89597/, the gains of doing padding
is negligible on the CPU architectures that we currently support while
potentially hurting DMA engine performances that may get involved with
the respective driver and/or hypervisor.

Signed-off-by: Costin Lupu <costin.lupu@xxxxxxxxx>
Signed-off-by: Simon Kuenzer <simon.kuenzer@xxxxxxxxx>
---
 include/arch/cc.h |  7 ++++++-
 uknetdev.c        | 42 ++++++++++++++++++------------------------
 2 files changed, 24 insertions(+), 25 deletions(-)

diff --git a/include/arch/cc.h b/include/arch/cc.h
index a1d0c34..0593f05 100644
--- a/include/arch/cc.h
+++ b/include/arch/cc.h
@@ -51,7 +51,12 @@
 
 /* 32 bit checksum calculation */
 #define LWIP_CHKSUM_ALGORITHM 3
-#define ETH_PAD_SIZE 2
+
+/*
+ * Disable padding on Ethernet frames
+ * (not supported by uknetdev driver)
+ */
+#define ETH_PAD_SIZE 0
 
 /* rand */
 #define LWIP_RAND() uk_swrand_randr()
diff --git a/uknetdev.c b/uknetdev.c
index 15464f0..ff61462 100644
--- a/uknetdev.c
+++ b/uknetdev.c
@@ -56,6 +56,22 @@
 
 #include <uk/essentials.h>
 
+/*
+ * NOTE: We do not support ETH_PAD_SIZE with uknetdev.
+ *       According to https://lwn.net/Articles/89597/, the gain of doing 
padding
+ *       seems to be negligible on the CPU architectures that we currently
+ *       support and may hurt DMA engine performances that may get involved 
with
+ *       the driver and/or hypervisor. Additionally, it will cause difficulties
+ *       with the current netfront driver because the protocol does not support
+ *       setting an offset for page-aligned receive buffers. Aligning the frame
+ *       could then only be achieved by moving the packet bytes in memory which
+ *       is obviously ways too costly. Because of this, we disable this feature
+ *       completely.
+ */
+#if ETH_PAD_SIZE
+#error ETH_PAD_SIZE is not '0'. This is not supported.
+#endif
+
 #define UKNETDEV_BPS 1000000000u
 #define UKNETDEV_BUFLEN 2048
 
@@ -97,8 +113,8 @@ UK_CTASSERT((sizeof(struct lwip_netdev_data)) <= 
CONFIG_UK_NETDEV_SCRATCH_SIZE);
  *       `struct uk_netdev` in order to avoid another allocation for these
  *       per-device fields.
  */
-static uint16_t rx_headroom = ETH_PAD_SIZE;
-static uint16_t tx_headroom = ETH_PAD_SIZE;
+static uint16_t rx_headroom = 0;
+static uint16_t tx_headroom = 0;
 
 #define netif_to_uknetdev(nf) \
        ((struct uk_netdev *) (nf)->state)
@@ -160,9 +176,6 @@ static err_t uknetdev_output(struct netif *nf, struct pbuf 
*p)
                return ERR_MEM;
        }
 
-#if ETH_PAD_SIZE
-       pbuf_header(p, -ETH_PAD_SIZE); /* drop the padding word */
-#endif /* ETH_PAD_SIZE */
        /*
         * Copy pbuf to netbuf
         * NOTE: Unfortunately, lwIP seems not to support zero-copy transmit,
@@ -174,26 +187,16 @@ static err_t uknetdev_output(struct netif *nf, struct 
pbuf *p)
                wpos += q->len;
        }
        nb->len = p->tot_len;
-#if ETH_PAD_SIZE
-       pbuf_header(p, ETH_PAD_SIZE); /* reclaim the padding word */
-#endif /* ETH_PAD_SIZE */
 
        /* Transmit packet */
        do {
                ret = uk_netdev_tx_one(dev, 0, nb);
        } while (uk_netdev_status_notready(ret));
        if (unlikely(ret < 0)) {
-#if ETH_PAD_SIZE
-               LWIP_DEBUGF(NETIF_DEBUG,
-                           ("%s: %c%c%u: Failed to send %"PRIu16" bytes\n",
-                            __func__, nf->name[0], nf->name[1], nf->num,
-                            p->tot_len - ETH_PAD_SIZE));
-#else /* ETH_PAD_SIZE */
                LWIP_DEBUGF(NETIF_DEBUG,
                            ("%s: %c%c%u: Failed to send %"PRIu16" bytes\n",
                             __func__, nf->name[0], nf->name[1], nf->num,
                             p->tot_len));
-#endif /* ETH_PAD_SIZE */
                /*
                 * Decrease refcount again because in
                 * the error case the netdev did not consume the pbuf
@@ -201,15 +204,9 @@ static err_t uknetdev_output(struct netif *nf, struct pbuf 
*p)
                uk_netbuf_free_single(nb);
                return ERR_IF;
        }
-#if ETH_PAD_SIZE
-       LWIP_DEBUGF(NETIF_DEBUG, ("%s: %c%c%u: Sent %"PRIu16" bytes\n",
-                                 __func__, nf->name[0], nf->name[1], nf->num,
-                                 p->tot_len - ETH_PAD_SIZE));
-#else /* ETH_PAD_SIZE */
        LWIP_DEBUGF(NETIF_DEBUG, ("%s: %c%c%u: Sent %"PRIu16" bytes\n",
                                  __func__, nf->name[0], nf->name[1], nf->num,
                                  p->tot_len));
-#endif /* ETH_PAD_SIZE */
 
        return ERR_OK;
 }
@@ -265,9 +262,6 @@ static void uknetdev_input(struct uk_netdev *dev,
                             nb->len));
 
                /* Send packet to lwip */
-#if ETH_PAD_SIZE
-               uk_netbuf_header(nb, ETH_PAD_SIZE);
-#endif /* ETH_PAD_SIZE */
                p = lwip_netbuf_to_pbuf(nb);
                p->payload = nb->data;
                p->tot_len = p->len = nb->len;
-- 
2.20.1