cachepc-linux

Fork of AMDESE/linux with modifications for CachePC side-channel attack
git clone https://git.sinitax.com/sinitax/cachepc-linux
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hash.h (2429B)


      1/* SPDX-License-Identifier: GPL-2.0 */
      2#ifndef _ASM_HASH_H
      3#define _ASM_HASH_H
      4
      5/*
      6 * Fortunately, most people who want to run Linux on Microblaze enable
      7 * both multiplier and barrel shifter, but omitting them is technically
      8 * a supported configuration.
      9 *
     10 * With just a barrel shifter, we can implement an efficient constant
     11 * multiply using shifts and adds.  GCC can find a 9-step solution, but
     12 * this 6-step solution was found by Yevgen Voronenko's implementation
     13 * of the Hcub algorithm at http://spiral.ece.cmu.edu/mcm/gen.html.
     14 *
     15 * That software is really not designed for a single multiplier this large,
     16 * but if you run it enough times with different seeds, it'll find several
     17 * 6-shift, 6-add sequences for computing x * 0x61C88647.  They are all
     18 *	c = (x << 19) + x;
     19 *	a = (x <<  9) + c;
     20 *	b = (x << 23) + a;
     21 *	return (a<<11) + (b<<6) + (c<<3) - b;
     22 * with variations on the order of the final add.
     23 *
     24 * Without even a shifter, it's hopless; any hash function will suck.
     25 */
     26
     27#if CONFIG_XILINX_MICROBLAZE0_USE_HW_MUL == 0
     28
     29#define HAVE_ARCH__HASH_32 1
     30
     31/* Multiply by GOLDEN_RATIO_32 = 0x61C88647 */
     32static inline u32 __attribute_const__ __hash_32(u32 a)
     33{
     34#if CONFIG_XILINX_MICROBLAZE0_USE_BARREL
     35	unsigned int b, c;
     36
     37	/* Phase 1: Compute three intermediate values */
     38	b =  a << 23;
     39	c = (a << 19) + a;
     40	a = (a <<  9) + c;
     41	b += a;
     42
     43	/* Phase 2: Compute (a << 11) + (b << 6) + (c << 3) - b */
     44	a <<= 5;
     45	a += b;		/* (a << 5) + b */
     46	a <<= 3;
     47	a += c;		/* (a << 8) + (b << 3) + c */
     48	a <<= 3;
     49	return a - b;	/* (a << 11) + (b << 6) + (c << 3) - b */
     50#else
     51	/*
     52	 * "This is really going to hurt."
     53	 *
     54	 * Without a barrel shifter, left shifts are implemented as
     55	 * repeated additions, and the best we can do is an optimal
     56	 * addition-subtraction chain.  This one is not known to be
     57	 * optimal, but at 37 steps, it's decent for a 31-bit multiplier.
     58	 *
     59	 * Question: given its size (37*4 = 148 bytes per instance),
     60	 * and slowness, is this worth having inline?
     61	 */
     62	unsigned int b, c, d;
     63
     64	b = a << 4;	/* 4    */
     65	c = b << 1;	/* 1  5 */
     66	b += a;		/* 1  6 */
     67	c += b;		/* 1  7 */
     68	c <<= 3;	/* 3 10 */
     69	c -= a;		/* 1 11 */
     70	d = c << 7;	/* 7 18 */
     71	d += b;		/* 1 19 */
     72	d <<= 8;	/* 8 27 */
     73	d += a;		/* 1 28 */
     74	d <<= 1;	/* 1 29 */
     75	d += b;		/* 1 30 */
     76	d <<= 6;	/* 6 36 */
     77	return d + c;	/* 1 37 total instructions*/
     78#endif
     79}
     80
     81#endif /* !CONFIG_XILINX_MICROBLAZE0_USE_HW_MUL */
     82#endif /* _ASM_HASH_H */