Subject: [PATCH v24 03/20] iov_iter: skip copy if src == dst for direct data placement

From: Aurelien Aptel <[email-masked]>
Date: Thu, 4 Apr 2024 12:37:00 +0000

From: Ben Ben-Ishay <[email-masked]>

When using direct data placement (DDP) the NIC could write the payload
directly into the destination buffer and constructs SKBs such that
they point to this data. To skip copies when SKB data already resides
in the destination buffer we check if (src == dst), and skip the copy
when it's true.

Signed-off-by: Ben Ben-Ishay <[email-masked]>
Signed-off-by: Boris Pismenny <[email-masked]>
Signed-off-by: Or Gerlitz <[email-masked]>
Signed-off-by: Yoray Zack <[email-masked]>
Signed-off-by: Shai Malin <[email-masked]>
Signed-off-by: Aurelien Aptel <[email-masked]>
Reviewed-by: Chaitanya Kulkarni <[email-masked]>
---
 lib/iov_iter.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 4a6a9f419bd7..a85125485174 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -62,7 +62,14 @@ static __always_inline
 size_t memcpy_to_iter(void *iter_to, size_t progress,
 		      size_t len, void *from, void *priv2)
 {
-	memcpy(iter_to, from + progress, len);
+	/*
+	 * When using direct data placement (DDP) the hardware writes
+	 * data directly to the destination buffer, and constructs
+	 * IOVs such that they point to this data.
+	 * Thus, when the src == dst we skip the memcpy.
+	 */
+	if (iter_to != from + progress)
+		memcpy(iter_to, from + progress, len);
 	return 0;
 }
 
-- 
2.34.1