netdev - Performance loss due to commit 37c3185 ([NET]: Added GSO toggle)

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Date:	Sat, 20 Jun 2015 11:55:03 +0200
From:	christophe leroy <christophe.leroy@....fr>
To:	Herbert Xu <herbert@...dor.apana.org.au>
CC:	netdev <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>
Subject: Performance loss due to commit  37c3185 ([NET]: Added GSO toggle)

Hello Herbert,

In commit "[NET]: Added GSO toggle" 
37c3185a02d4b85fbe134bf5204535405dd2c957,
you force NETIF_F_HW_CSUM if GSO feature is selected.
By default, SW GSO is active as soon as a network board has NETIF_F_SG 
feature.
This means that function sk_setup_caps() forces NETIF_F_HW_CSUM for any 
board having NETIF_F_SG

For boards having no HW checksum capability, this results in performance 
loss due to data copy being done in skb_do_copy_data_nocache() with 
copy_from_user() then checksum being done later with csum_partial() 
instead of getting both done at the same time using 
csum_and_copy_from_user()

Is there a reason for forcing NETIF_F_HW_CSUM   ?

Christophe

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