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Subject[PATCH V5 3/6 net-next] skbuff: Add userspace zero-copy buffers in skb
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This patch adds userspace buffers support in skb shared info. A new 
struct skb_ubuf_info is needed to maintain the userspace buffers
argument and index, a callback is used to notify userspace to release
the buffers once lower device has done DMA (Last reference to that skb
has gone).

Signed-off-by: Shirley Ma <xma@us.ibm.com>
---

include/linux/skbuff.h | 26 ++++++++++++++++++++++++++
net/core/skbuff.c | 13 +++++++++++++
2 files changed, 39 insertions(+), 0 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 79aafbb..40faffe 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -189,6 +189,18 @@ enum {
SKBTX_DRV_NEEDS_SK_REF = 1 << 3,
};

+/*
+ * The callback notifies userspace to release buffers when skb DMA is
done in
+ * lower device, the skb last reference should be 0 when calling this.
+ * The desc is used to track userspace buffer index.
+ */
+struct skb_ubuf_info {
+ /* support buffers allocation from userspace */
+ void (*callback)(struct sk_buff *);
+ void *arg;
+ size_t desc;
+};
+
/* This data is invariant across clones and lives at
* the end of the header data, ie. at skb->end.
*/
@@ -211,6 +223,10 @@ struct skb_shared_info {
/* Intermediate layers must ensure that destructor_arg
* remains valid until skb destructor */
void * destructor_arg;
+
+ /* DMA mapping from/to userspace buffers */
+ struct skb_ubuf_info ubuf;
+
/* must be last field, see pskb_expand_head() */
skb_frag_t frags[MAX_SKB_FRAGS];
};
@@ -2261,5 +2277,15 @@ static inline void
skb_checksum_none_assert(struct sk_buff *skb)
}

bool skb_partial_csum_set(struct sk_buff *skb, u16 start, u16 off);
+
+/*
+ * skb_ubuf - is the buffer from userspace
+ * @skb: buffer to check
+ */
+static inline int skb_ubuf(const struct sk_buff *skb)
+{
+ return (skb_shinfo(skb)->ubuf.callback != NULL);
+}
+
#endif /* __KERNEL__ */
#endif /* _LINUX_SKBUFF_H */
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 7ebeed0..9cbd3fc 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -210,6 +210,8 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t
gfp_mask,
shinfo = skb_shinfo(skb);
memset(shinfo, 0, offsetof(struct skb_shared_info, dataref));
atomic_set(&shinfo->dataref, 1);
+ shinfo->ubuf.callback = NULL;
+ shinfo->ubuf.arg = NULL;
kmemcheck_annotate_variable(shinfo->destructor_arg);

if (fclone) {
@@ -328,6 +330,14 @@ static void skb_release_data(struct sk_buff *skb)
put_page(skb_shinfo(skb)->frags[i].page);
}

+ /*
+ * if skb buf is from userspace, we need to notify the caller
+ * the lower device DMA has done;
+ */
+ if (skb_shinfo(skb)->ubuf.callback) {
+ skb_shinfo(skb)->ubuf.callback(skb);
+ skb_shinfo(skb)->ubuf.callback = NULL;
+ }
if (skb_has_frag_list(skb))
skb_drop_fraglist(skb);

@@ -480,6 +490,9 @@ bool skb_recycle_check(struct sk_buff *skb, int
skb_size)
if (irqs_disabled())
return false;

+ if (skb_ubuf(skb))
+ return false;
+
if (skb_is_nonlinear(skb) || skb->fclone != SKB_FCLONE_UNAVAILABLE)
return false;




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