linux-kernel - RE: [RFC] csum experts, csum

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Date:	Mon, 24 Mar 2014 07:13:42 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	David Miller <davem@...emloft.net>,
	"andi@...stfloor.org" <andi@...stfloor.org>,
	"hpa@...or.com" <hpa@...or.com>,
	"kaber@...sh.net" <kaber@...sh.net>,
	"herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>,
	"hkchu@...gle.com" <hkchu@...gle.com>,
	"mwdalton@...gle.com" <mwdalton@...gle.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [RFC] csum experts, csum_replace2() is too expensive

On Mon, 2014-03-24 at 06:17 -0700, Eric Dumazet wrote:
> On Mon, 2014-03-24 at 10:30 +0000, David Laight wrote:

> > ip_fast_csum() either needs an explicit "m" constraint for the actual
> > buffer (and target) bytes, or the stronger "memory" constraint.
> > The 'volatile' is then not needed.

I am testing the following :

diff --git a/arch/x86/include/asm/checksum_64.h b/arch/x86/include/asm/checksum_64.h
index e6fd8a026c7b..89d7fa8837b5 100644
--- a/arch/x86/include/asm/checksum_64.h
+++ b/arch/x86/include/asm/checksum_64.h
@@ -45,6 +45,9 @@ static inline __sum16 csum_fold(__wsum sum)
 static inline __sum16 ip_fast_csum(const void *iph, unsigned int ihl)
 {
 	unsigned int sum;
+	struct full_ip_header {
+		unsigned int w[ihl];
+	};
 
 	asm("  movl (%1), %0\n"
 	    "  subl $4, %2\n"
@@ -67,8 +70,9 @@ static inline __sum16 ip_fast_csum(const void *iph, unsigned int ihl)
 	   are modified, we must also specify them as outputs, or gcc
 	   will assume they contain their original values. */
 	    : "=r" (sum), "=r" (iph), "=r" (ihl)
-	    : "1" (iph), "2" (ihl)
-	    : "memory");
+	    : "1" (iph), "2" (ihl),
+	      "m" (*(struct full_ip_header *) iph)
+	);
 	return (__force __sum16)sum;
 }
 


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