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| author | Samuel Thibault <samuel.thibault@ens-lyon.org> | 2023-02-19 22:24:32 +0100 |
|---|---|---|
| committer | Samuel Thibault <samuel.thibault@ens-lyon.org> | 2023-02-19 22:26:47 +0100 |
| commit | 67efb746c492c25fac4d77aa16b808a8aa26089d (patch) | |
| tree | 3031425a1909281b06a0b69600bb4f00cb0c3ca2 /pfinet | |
| parent | b8bcd4eb1438721f6a52bfd1e458307d4e105d89 (diff) | |
| download | hurd-67efb746c492c25fac4d77aa16b808a8aa26089d.tar.gz hurd-67efb746c492c25fac4d77aa16b808a8aa26089d.tar.bz2 hurd-67efb746c492c25fac4d77aa16b808a8aa26089d.zip | |
pfinet: Align packets
The Ethernet header is 14 bytes long, and thus leads to IP header
misalignment. This uses skb_reserve to introduce 2 bytes of padding to
realign IP headers.
Diffstat (limited to 'pfinet')
| -rw-r--r-- | pfinet/ethernet.c | 3 | ||||
| -rw-r--r-- | pfinet/linux-src/include/linux/skbuff.h | 24 | ||||
| -rw-r--r-- | pfinet/tunnel.c | 3 |
3 files changed, 28 insertions, 2 deletions
diff --git a/pfinet/ethernet.c b/pfinet/ethernet.c index c317820c..82c318b2 100644 --- a/pfinet/ethernet.c +++ b/pfinet/ethernet.c @@ -153,7 +153,8 @@ ethernet_demuxer (mach_msg_header_t *inp, + msg->packet_type.msgt_number - sizeof (struct packet_header); pthread_mutex_lock (&net_bh_lock); - skb = alloc_skb (datalen, GFP_ATOMIC); + skb = alloc_skb (NET_IP_ALIGN + datalen, GFP_ATOMIC); + skb_reserve(skb, NET_IP_ALIGN); skb_put (skb, datalen); skb->dev = dev; diff --git a/pfinet/linux-src/include/linux/skbuff.h b/pfinet/linux-src/include/linux/skbuff.h index 00f9ab2a..46eb995e 100644 --- a/pfinet/linux-src/include/linux/skbuff.h +++ b/pfinet/linux-src/include/linux/skbuff.h @@ -498,6 +498,30 @@ static __inline__ int skb_tailroom(struct sk_buff *skb) return skb->end-skb->tail; } +/* + * CPUs often take a performance hit when accessing unaligned memory + * locations. The actual performance hit varies, it can be small if the + * hardware handles it or large if we have to take an exception and fix it + * in software. + * + * Since an ethernet header is 14 bytes network drivers often end up with + * the IP header at an unaligned offset. The IP header can be aligned by + * shifting the start of the packet by 2 bytes. Drivers should do this + * with: + * + * skb_reserve(skb, NET_IP_ALIGN); + * + * The downside to this alignment of the IP header is that the DMA is now + * unaligned. On some architectures the cost of an unaligned DMA is high + * and this cost outweighs the gains made by aligning the IP header. + * + * Since this trade off varies between architectures, we allow NET_IP_ALIGN + * to be overridden. + */ +#ifndef NET_IP_ALIGN +#define NET_IP_ALIGN 2 +#endif + static __inline__ void skb_reserve(struct sk_buff *skb, unsigned int len) { skb->data+=len; diff --git a/pfinet/tunnel.c b/pfinet/tunnel.c index e11ab670..b519ebd1 100644 --- a/pfinet/tunnel.c +++ b/pfinet/tunnel.c @@ -391,7 +391,8 @@ trivfs_S_io_write (struct trivfs_protid *cred, pthread_mutex_lock (&tdev->lock); pthread_mutex_lock (&net_bh_lock); - skb = alloc_skb (datalen, GFP_ATOMIC); + skb = alloc_skb (NET_IP_ALIGN + datalen, GFP_ATOMIC); + skb_reserve(skb, NET_IP_ALIGN); skb->len = datalen; skb->dev = &tdev->dev; |