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From: Peter Zijlstra <peterz@infradead.org>
To: Eric Dumazet <eric.dumazet@gmail.com>
Cc: "David S . Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>, netdev <netdev@vger.kernel.org>,
	Eric Dumazet <edumazet@google.com>,
	x86@kernel.org, Alexander Duyck <alexander.duyck@gmail.com>
Subject: Re: [RFC] x86/csum: rewrite csum_partial()
Date: Thu, 11 Nov 2021 10:44:19 +0100	[thread overview]
Message-ID: <YYzl8/7N+Tv/j0RV@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <YYzd+zdzqUM5/ZKL@hirez.programming.kicks-ass.net>

On Thu, Nov 11, 2021 at 10:10:19AM +0100, Peter Zijlstra wrote:
> On Wed, Nov 10, 2021 at 10:53:22PM -0800, Eric Dumazet wrote:
> > +		/*
> > +		 * This implements an optimized version of
> > +		 * switch (dwords) {
> > +		 * case 15: res = add_with_carry(res, buf32[14]); fallthrough;
> > +		 * case 14: res = add_with_carry(res, buf32[13]); fallthrough;
> > +		 * case 13: res = add_with_carry(res, buf32[12]); fallthrough;
> > +		 * ...
> > +		 * case 3: res = add_with_carry(res, buf32[2]); fallthrough;
> > +		 * case 2: res = add_with_carry(res, buf32[1]); fallthrough;
> > +		 * case 1: res = add_with_carry(res, buf32[0]); fallthrough;
> > +		 * }
> > +		 *
> > +		 * "adcl 8byteoff(%reg1),%reg2" are using either 3 or 4 bytes.
> > +		 */
> > +		asm("	call 1f\n"
> > +		    "1:	pop %[dest]\n"
> 
> That's terrible. I think on x86_64 we can do: lea (%%rip), %[dest], not
> sure what would be the best way on i386.
> 
> > +		    "	lea (2f-1b)(%[dest],%[skip],4),%[dest]\n"
> > +		    "	clc\n"
> > +		    "	jmp *%[dest]\n               .align 4\n"
> 
> That's an indirect branch, you can't do that these days. This would need
> to use JMP_NOSPEC (except we don't have a !ASSEMBLER version of that.
> But that would also completely and utterly destroy performance.
> 
> Also, objtool would complain about this if it hadn't tripped over that
> first instruction:
> 
>  arch/x86/lib/csum-partial_64.o: warning: objtool: do_csum()+0x84: indirect jump found in RETPOLINE build
> 
> I'm not sure what the best way is to unroll loops without using computed
> gotos/jump-tables though :/
> 
> > +		    "2:\n"
> > +		    "	adcl 14*4(%[src]),%[res]\n   .align 4\n"
> > +		    "	adcl 13*4(%[src]),%[res]\n   .align 4\n"
> > +		    "	adcl 12*4(%[src]),%[res]\n   .align 4\n"
> > +		    "	adcl 11*4(%[src]),%[res]\n   .align 4\n"
> > +		    "	adcl 10*4(%[src]),%[res]\n   .align 4\n"
> > +		    "	adcl 9*4(%[src]),%[res]\n   .align 4\n"
> > +		    "	adcl 8*4(%[src]),%[res]\n   .align 4\n"
> > +		    "	adcl 7*4(%[src]),%[res]\n   .align 4\n"
> > +		    "	adcl 6*4(%[src]),%[res]\n   .align 4\n"
> > +		    "	adcl 5*4(%[src]),%[res]\n   .align 4\n"
> > +		    "	adcl 4*4(%[src]),%[res]\n   .align 4\n"
> > +		    "	adcl 3*4(%[src]),%[res]\n   .align 4\n"
> > +		    "	adcl 2*4(%[src]),%[res]\n   .align 4\n"
> > +		    "	adcl 1*4(%[src]),%[res]\n   .align 4\n"
> > +		    "	adcl 0*4(%[src]),%[res]\n"
> > +		    "	adcl $0,%[res]"
> 
> If only the CPU would accept: REP ADCL (%%rsi), %[res]   :/
> 
> > +			: [res] "=r" (result), [dest] "=&r" (dest)
> > +			: [src] "r" (buff), "[res]" (result),
> > +			  [skip] "r" (dwords ^ 15)
> > +			: "memory");
> > +	}

This is the best I could come up with ...

--- a/arch/x86/lib/csum-partial_64.c
+++ b/arch/x86/lib/csum-partial_64.c
@@ -32,7 +32,6 @@ static inline unsigned short from32to16(
  */
 static unsigned do_csum(const unsigned char *buff, unsigned len)
 {
-	unsigned long dwords;
 	unsigned odd, result = 0;
 
 	odd = 1 & (unsigned long) buff;
@@ -64,50 +63,54 @@ static unsigned do_csum(const unsigned c
 		result = add32_with_carry(temp64 >> 32, temp64 & 0xffffffff);
 	}
 
-	dwords = len >> 2;
-	if (dwords) { /* dwords is in [1..15] */
-		unsigned long dest;
-
-		/*
-		 * This implements an optimized version of
-		 * switch (dwords) {
-		 * case 15: res = add_with_carry(res, buf32[14]); fallthrough;
-		 * case 14: res = add_with_carry(res, buf32[13]); fallthrough;
-		 * case 13: res = add_with_carry(res, buf32[12]); fallthrough;
-		 * ...
-		 * case 3: res = add_with_carry(res, buf32[2]); fallthrough;
-		 * case 2: res = add_with_carry(res, buf32[1]); fallthrough;
-		 * case 1: res = add_with_carry(res, buf32[0]); fallthrough;
-		 * }
-		 *
-		 * "adcl 8byteoff(%reg1),%reg2" are using either 3 or 4 bytes.
-		 */
-		asm("	call 1f\n"
-		    "1:	pop %[dest]\n"
-		    "	lea (2f-1b)(%[dest],%[skip],4),%[dest]\n"
-		    "	clc\n"
-		    "	jmp *%[dest]\n               .align 4\n"
-		    "2:\n"
-		    "	adcl 14*4(%[src]),%[res]\n   .align 4\n"
-		    "	adcl 13*4(%[src]),%[res]\n   .align 4\n"
-		    "	adcl 12*4(%[src]),%[res]\n   .align 4\n"
-		    "	adcl 11*4(%[src]),%[res]\n   .align 4\n"
-		    "	adcl 10*4(%[src]),%[res]\n   .align 4\n"
-		    "	adcl 9*4(%[src]),%[res]\n   .align 4\n"
-		    "	adcl 8*4(%[src]),%[res]\n   .align 4\n"
-		    "	adcl 7*4(%[src]),%[res]\n   .align 4\n"
-		    "	adcl 6*4(%[src]),%[res]\n   .align 4\n"
-		    "	adcl 5*4(%[src]),%[res]\n   .align 4\n"
-		    "	adcl 4*4(%[src]),%[res]\n   .align 4\n"
-		    "	adcl 3*4(%[src]),%[res]\n   .align 4\n"
-		    "	adcl 2*4(%[src]),%[res]\n   .align 4\n"
-		    "	adcl 1*4(%[src]),%[res]\n   .align 4\n"
-		    "	adcl 0*4(%[src]),%[res]\n"
-		    "	adcl $0,%[res]"
-			: [res] "=r" (result), [dest] "=&r" (dest)
-			: [src] "r" (buff), "[res]" (result),
-			  [skip] "r" (dwords ^ 15)
-			: "memory");
+	if (len >> 2) { /* dwords is in [1..15] */
+		if (len >= 32) {
+			asm("	addl 0*4(%[src]),%[res]\n"
+			    "	adcl 1*4(%[src]),%[res]\n"
+			    "	adcl 2*4(%[src]),%[res]\n"
+			    "	adcl 3*4(%[src]),%[res]\n"
+			    "	adcl 4*4(%[src]),%[res]\n"
+			    "	adcl 5*4(%[src]),%[res]\n"
+			    "	adcl 6*4(%[src]),%[res]\n"
+			    "	adcl 7*4(%[src]),%[res]\n"
+			    "	adcl $0,%[res]"
+			    : [res] "=r" (result)
+			    : [src] "r" (buff), "[res]" (result)
+			    : "memory");
+			buff += 32;
+			len -= 32;
+		}
+		if (len >= 16) {
+			asm("	addl 0*4(%[src]),%[res]\n"
+			    "	adcl 1*4(%[src]),%[res]\n"
+			    "	adcl 2*4(%[src]),%[res]\n"
+			    "	adcl 3*4(%[src]),%[res]\n"
+			    "	adcl $0,%[res]"
+			    : [res] "=r" (result)
+			    : [src] "r" (buff), "[res]" (result)
+			    : "memory");
+			buff += 16;
+			len -= 16;
+		}
+		if (len >= 8) {
+			asm("	addl 0*4(%[src]),%[res]\n"
+			    "	adcl 1*4(%[src]),%[res]\n"
+			    "	adcl $0,%[res]"
+			    : [res] "=r" (result)
+			    : [src] "r" (buff), "[res]" (result)
+			    : "memory");
+			buff += 8;
+			len -= 8;
+		}
+		if (len >= 4) {
+			asm("	addl 0*4(%[src]),%[res]\n"
+			    "	adcl $0,%[res]"
+			    : [res] "=r" (result)
+			    : [src] "r" (buff), "[res]" (result)
+			    : "memory");
+			buff += 4;
+			len -= 4;
+		}
 	}
 
 	if (len & 3U) {
@@ -120,7 +123,7 @@ static unsigned do_csum(const unsigned c
 		if (len & 1)
 			result += *buff;
 	}
-	if (unlikely(odd)) { 
+	if (unlikely(odd)) {
 		result = from32to16(result);
 		result = ((result >> 8) & 0xff) | ((result & 0xff) << 8);
 	}

  reply	other threads:[~2021-11-11  9:44 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-11-11  6:53 [RFC] x86/csum: rewrite csum_partial() Eric Dumazet
2021-11-11  9:10 ` Peter Zijlstra
2021-11-11  9:44   ` Peter Zijlstra [this message]
2021-11-11 16:02     ` Eric Dumazet
2021-11-11 16:52       ` Eric Dumazet
2021-11-11 18:17         ` Andrew Cooper
2021-11-11 19:02           ` Eric Dumazet
2021-11-11 16:51 ` Alexander Duyck
2021-11-14 13:07 ` David Laight
2021-11-14 14:12   ` David Laight
2021-11-15 10:23     ` David Laight

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