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

[Simon Kuenzer <simon.kuenzer@xxxxxxxxx>]

Hey Sharan, hey Costin,

as Costin discussed with me offline, we will remove his authorship from 
this patch while upstreaming. @Costin, can you give your okay to this 
mail? It is just to have a public confirmation.
Thanks a lot,

Simon

On 09.11.20 17:06, Simon Kuenzer wrote:
> 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;