netdev - [PATCH v16 03/20] iov_iter: skip copy if src == dst for direct data placement

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Message-Id: <20230928150954.1684-4-aaptel@nvidia.com>
Date: Thu, 28 Sep 2023 15:09:37 +0000
From: Aurelien Aptel <aaptel@...dia.com>
To: linux-nvme@...ts.infradead.org,
	netdev@...r.kernel.org,
	sagi@...mberg.me,
	hch@....de,
	kbusch@...nel.org,
	axboe@...com,
	chaitanyak@...dia.com,
	davem@...emloft.net,
	kuba@...nel.org
Cc: Ben Ben-Ishay <benishay@...dia.com>,
	aaptel@...dia.com,
	aurelien.aptel@...il.com,
	smalin@...dia.com,
	malin1024@...il.com,
	ogerlitz@...dia.com,
	yorayz@...dia.com,
	borisp@...dia.com,
	galshalom@...dia.com,
	mgurtovoy@...dia.com,
	viro@...iv.linux.org.uk
Subject: [PATCH v16 03/20] iov_iter: skip copy if src == dst for direct data placement

From: Ben Ben-Ishay <benishay@...dia.com>

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 <benishay@...dia.com>
Signed-off-by: Boris Pismenny <borisp@...dia.com>
Signed-off-by: Or Gerlitz <ogerlitz@...dia.com>
Signed-off-by: Yoray Zack <yorayz@...dia.com>
Signed-off-by: Shai Malin <smalin@...dia.com>
Signed-off-by: Aurelien Aptel <aaptel@...dia.com>
Reviewed-by: Chaitanya Kulkarni <kch@...dia.com>
---
 lib/iov_iter.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 27234a820eeb..279b2c5b1936 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -313,9 +313,15 @@ size_t _copy_to_iter(const void *addr, size_t bytes, struct iov_iter *i)
 		return 0;
 	if (user_backed_iter(i))
 		might_fault();
+	/*
+	 * 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.
+	 */
 	iterate_and_advance(i, bytes, base, len, off,
 		copyout(base, addr + off, len),
-		memcpy(base, addr + off, len)
+		(base != addr + off) && memcpy(base, addr + off, len)
 	)
 
 	return bytes;
-- 
2.34.1