Now available: The Design and Implementation of the FreeBSD Operating System (Second Edition)
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FreeBSD/Linux Kernel Cross Reference
sys/i386/i386/in_cksum.c
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1 /*- 2 * Copyright (c) 1990 The Regents of the University of California. 3 * All rights reserved. 4 * 5 * Redistribution and use in source and binary forms, with or without 6 * modification, are permitted provided that the following conditions 7 * are met: 8 * 1. Redistributions of source code must retain the above copyright 9 * notice, this list of conditions and the following disclaimer. 10 * 2. Redistributions in binary form must reproduce the above copyright 11 * notice, this list of conditions and the following disclaimer in the 12 * documentation and/or other materials provided with the distribution. 13 * 3. All advertising materials mentioning features or use of this software 14 * must display the following acknowledgement: 15 * This product includes software developed by the University of 16 * California, Berkeley and its contributors. 17 * 4. Neither the name of the University nor the names of its contributors 18 * may be used to endorse or promote products derived from this software 19 * without specific prior written permission. 20 * 21 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 22 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 23 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 24 * ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 25 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 26 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 27 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 28 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 29 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 30 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 31 * SUCH DAMAGE. 32 * 33 * from tahoe: in_cksum.c 1.2 86/01/05 34 * from: @(#)in_cksum.c 1.3 (Berkeley) 1/19/91 35 * $FreeBSD: src/sys/i386/i386/in_cksum.c,v 1.8.4.2 1999/09/05 08:11:08 peter Exp $ 36 */ 37 38 #include <sys/param.h> 39 #include <sys/systm.h> 40 #include <sys/mbuf.h> 41 42 extern int in_cksum __P((struct mbuf *m, int len)); 43 44 /* 45 * Checksum routine for Internet Protocol family headers. 46 * 47 * This routine is very heavily used in the network 48 * code and should be modified for each CPU to be as fast as possible. 49 * 50 * This implementation is 386 version. 51 */ 52 53 #undef ADDCARRY 54 #define ADDCARRY(x) if ((x) > 0xffff) (x) -= 0xffff 55 #define REDUCE {sum = (sum & 0xffff) + (sum >> 16); ADDCARRY(sum);} 56 57 /* 58 * Thanks to gcc we don't have to guess 59 * which registers contain sum & w. 60 */ 61 #define ADD(n) __asm("addl " #n "(%2), %0" : "=r" (sum) : "" (sum), "r" (w)) 62 #define ADDC(n) __asm("adcl " #n "(%2), %0" : "=r" (sum) : "" (sum), "r" (w)) 63 #define LOAD(n) __asm volatile("movb " #n "(%1), %0" : "=r" (junk) : "r" (w)) 64 #define MOP __asm("adcl $0, %0" : "=r" (sum) : "" (sum)) 65 66 int 67 in_cksum(m, len) 68 register struct mbuf *m; 69 register int len; 70 { 71 register u_short *w; 72 register unsigned sum = 0; 73 register int mlen = 0; 74 int byte_swapped = 0; 75 union { char c[2]; u_short s; } su; 76 77 for (;m && len; m = m->m_next) { 78 if (m->m_len == 0) 79 continue; 80 w = mtod(m, u_short *); 81 if (mlen == -1) { 82 /* 83 * The first byte of this mbuf is the continuation 84 * of a word spanning between this mbuf and the 85 * last mbuf. 86 */ 87 88 /* su.c[0] is already saved when scanning previous 89 * mbuf. sum was REDUCEd when we found mlen == -1 90 */ 91 su.c[1] = *(u_char *)w; 92 sum += su.s; 93 w = (u_short *)((char *)w + 1); 94 mlen = m->m_len - 1; 95 len--; 96 } else 97 mlen = m->m_len; 98 if (len < mlen) 99 mlen = len; 100 len -= mlen; 101 /* 102 * Force to long boundary so we do longword aligned 103 * memory operations 104 */ 105 if (3 & (int) w) { 106 REDUCE; 107 if ((1 & (int) w) && (mlen > 0)) { 108 sum <<= 8; 109 su.c[0] = *(char *)w; 110 w = (u_short *)((char *)w + 1); 111 mlen--; 112 byte_swapped = 1; 113 } 114 if ((2 & (int) w) && (mlen >= 2)) { 115 sum += *w++; 116 mlen -= 2; 117 } 118 } 119 /* 120 * Advance to a 486 cache line boundary. 121 */ 122 if (4 & (int) w && mlen >= 4) { 123 ADD(0); 124 MOP; 125 w += 2; 126 mlen -= 4; 127 } 128 if (8 & (int) w && mlen >= 8) { 129 ADD(0); 130 ADDC(4); 131 MOP; 132 w += 4; 133 mlen -= 8; 134 } 135 /* 136 * Do as much of the checksum as possible 32 bits at at time. 137 * In fact, this loop is unrolled to make overhead from 138 * branches &c small. 139 */ 140 mlen -= 1; 141 while ((mlen -= 32) >= 0) { 142 u_char junk; 143 /* 144 * Add with carry 16 words and fold in the last 145 * carry by adding a 0 with carry. 146 * 147 * The early ADD(16) and the LOAD(32) are to load 148 * the next 2 cache lines in advance on 486's. The 149 * 486 has a penalty of 2 clock cycles for loading 150 * a cache line, plus whatever time the external 151 * memory takes to load the first word(s) addressed. 152 * These penalties are unavoidable. Subsequent 153 * accesses to a cache line being loaded (and to 154 * other external memory?) are delayed until the 155 * whole load finishes. These penalties are mostly 156 * avoided by not accessing external memory for 157 * 8 cycles after the ADD(16) and 12 cycles after 158 * the LOAD(32). The loop terminates when mlen 159 * is initially 33 (not 32) to guaranteed that 160 * the LOAD(32) is within bounds. 161 */ 162 ADD(16); 163 ADDC(0); 164 ADDC(4); 165 ADDC(8); 166 ADDC(12); 167 LOAD(32); 168 ADDC(20); 169 ADDC(24); 170 ADDC(28); 171 MOP; 172 w += 16; 173 } 174 mlen += 32 + 1; 175 if (mlen >= 32) { 176 ADD(16); 177 ADDC(0); 178 ADDC(4); 179 ADDC(8); 180 ADDC(12); 181 ADDC(20); 182 ADDC(24); 183 ADDC(28); 184 MOP; 185 w += 16; 186 mlen -= 32; 187 } 188 if (mlen >= 16) { 189 ADD(0); 190 ADDC(4); 191 ADDC(8); 192 ADDC(12); 193 MOP; 194 w += 8; 195 mlen -= 16; 196 } 197 if (mlen >= 8) { 198 ADD(0); 199 ADDC(4); 200 MOP; 201 w += 4; 202 mlen -= 8; 203 } 204 if (mlen == 0 && byte_swapped == 0) 205 continue; /* worth 1% maybe ?? */ 206 REDUCE; 207 while ((mlen -= 2) >= 0) { 208 sum += *w++; 209 } 210 if (byte_swapped) { 211 sum <<= 8; 212 byte_swapped = 0; 213 if (mlen == -1) { 214 su.c[1] = *(char *)w; 215 sum += su.s; 216 mlen = 0; 217 } else 218 mlen = -1; 219 } else if (mlen == -1) 220 /* 221 * This mbuf has odd number of bytes. 222 * There could be a word split betwen 223 * this mbuf and the next mbuf. 224 * Save the last byte (to prepend to next mbuf). 225 */ 226 su.c[0] = *(char *)w; 227 } 228 229 if (len) 230 printf("cksum: out of data\n"); 231 if (mlen == -1) { 232 /* The last mbuf has odd # of bytes. Follow the 233 standard (the odd byte is shifted left by 8 bits) */ 234 su.c[1] = 0; 235 sum += su.s; 236 } 237 REDUCE; 238 return (~sum & 0xffff); 239 }
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