[PATCH 1/2] net: Add function to get SW rxhash

[PATCH 1/2] net: Add function to get SW rxhash

[Tom Herbert]

Some uses of skb_get_rxhash expect that the function will return
a consistent value whether it is called on RX or TX paths. On RX
path, we will use the rxhash if provided by the NIC, so this
would not normally be the same result computed in TX path which is
a software calculation.

This patch adds skb_get_sw_rxhash to explicitly request a hash
calculated by the stack, disregarding the hash provided by NIC.

Signed-off-by: Tom Herbert <therbert@google.com>
---
 include/linux/skbuff.h    | 11 ++++++++++-
 net/core/flow_dissector.c |  1 +
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 2ddb48d..917a590 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -488,6 +488,7 @@ struct sk_buff {
 	__u8			pfmemalloc:1;
 	__u8			ooo_okay:1;
 	__u8			l4_rxhash:1;
+	__u8			sw_rxhash:1;
 	__u8			wifi_acked_valid:1;
 	__u8			wifi_acked:1;
 	__u8			no_fcs:1;
@@ -498,7 +499,7 @@ struct sk_buff {
 	 * headers if needed
 	 */
 	__u8			encapsulation:1;
-	/* 7/9 bit hole (depending on ndisc_nodetype presence) */
+	/* 6/8 bit hole (depending on ndisc_nodetype presence) */
 	kmemcheck_bitfield_end(flags2);
 
 #if defined CONFIG_NET_DMA || defined CONFIG_NET_RX_BUSY_POLL
@@ -726,6 +727,14 @@ static inline __u32 skb_get_rxhash(struct sk_buff *skb)
 	return skb->rxhash;
 }
 
+static inline __u32 skb_get_sw_rxhash(struct sk_buff *skb)
+{
+	if (!skb->l4_rxhash && !skb->sw_rxhash)
+		__skb_get_rxhash(skb);
+
+	return skb->rxhash;
+}
+
 #ifdef NET_SKBUFF_DATA_USES_OFFSET
 static inline unsigned char *skb_end_pointer(const struct sk_buff *skb)
 {
diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
index 1929af8..8979121 100644
--- a/net/core/flow_dissector.c
+++ b/net/core/flow_dissector.c
@@ -200,6 +200,7 @@ void __skb_get_rxhash(struct sk_buff *skb)
 		hash = 1;
 
 	skb->rxhash = hash;
+	skb->sw_rxhash = 1;
 }
 EXPORT_SYMBOL(__skb_get_rxhash);
 
-- 
1.8.4

Re: [PATCH 1/2] net: Add function to get SW rxhash

[Eric Dumazet]

On Tue, 2013-09-24 at 09:20 -0700, Tom Herbert wrote:
> Some uses of skb_get_rxhash expect that the function will return
> a consistent value whether it is called on RX or TX paths. On RX
> path, we will use the rxhash if provided by the NIC, so this
> would not normally be the same result computed in TX path which is
> a software calculation.
> 
> This patch adds skb_get_sw_rxhash to explicitly request a hash
> calculated by the stack, disregarding the hash provided by NIC.
> 
> Signed-off-by: Tom Herbert <therbert@google.com>
> ---

1) Adding another skb field for this corner case, is it worth it ?
   Computing rxhash in software should be real fast for the
   cases where we really care of symmetry.

  ie ignore skb->rxhash and use __skb_get_rxhash()

2) Not copying sw_rxhash in __copy_skb_header()

   -> value will be undefined after a clone()