cachepc-linux

Fork of AMDESE/linux with modifications for CachePC side-channel attack
git clone https://git.sinitax.com/sinitax/cachepc-linux
Log | Files | Refs | README | LICENSE | sfeed.txt

win_minmax.c (3436B)


      1// SPDX-License-Identifier: GPL-2.0
      2/**
      3 * lib/minmax.c: windowed min/max tracker
      4 *
      5 * Kathleen Nichols' algorithm for tracking the minimum (or maximum)
      6 * value of a data stream over some fixed time interval.  (E.g.,
      7 * the minimum RTT over the past five minutes.) It uses constant
      8 * space and constant time per update yet almost always delivers
      9 * the same minimum as an implementation that has to keep all the
     10 * data in the window.
     11 *
     12 * The algorithm keeps track of the best, 2nd best & 3rd best min
     13 * values, maintaining an invariant that the measurement time of
     14 * the n'th best >= n-1'th best. It also makes sure that the three
     15 * values are widely separated in the time window since that bounds
     16 * the worse case error when that data is monotonically increasing
     17 * over the window.
     18 *
     19 * Upon getting a new min, we can forget everything earlier because
     20 * it has no value - the new min is <= everything else in the window
     21 * by definition and it's the most recent. So we restart fresh on
     22 * every new min and overwrites 2nd & 3rd choices. The same property
     23 * holds for 2nd & 3rd best.
     24 */
     25#include <linux/module.h>
     26#include <linux/win_minmax.h>
     27
     28/* As time advances, update the 1st, 2nd, and 3rd choices. */
     29static u32 minmax_subwin_update(struct minmax *m, u32 win,
     30				const struct minmax_sample *val)
     31{
     32	u32 dt = val->t - m->s[0].t;
     33
     34	if (unlikely(dt > win)) {
     35		/*
     36		 * Passed entire window without a new val so make 2nd
     37		 * choice the new val & 3rd choice the new 2nd choice.
     38		 * we may have to iterate this since our 2nd choice
     39		 * may also be outside the window (we checked on entry
     40		 * that the third choice was in the window).
     41		 */
     42		m->s[0] = m->s[1];
     43		m->s[1] = m->s[2];
     44		m->s[2] = *val;
     45		if (unlikely(val->t - m->s[0].t > win)) {
     46			m->s[0] = m->s[1];
     47			m->s[1] = m->s[2];
     48			m->s[2] = *val;
     49		}
     50	} else if (unlikely(m->s[1].t == m->s[0].t) && dt > win/4) {
     51		/*
     52		 * We've passed a quarter of the window without a new val
     53		 * so take a 2nd choice from the 2nd quarter of the window.
     54		 */
     55		m->s[2] = m->s[1] = *val;
     56	} else if (unlikely(m->s[2].t == m->s[1].t) && dt > win/2) {
     57		/*
     58		 * We've passed half the window without finding a new val
     59		 * so take a 3rd choice from the last half of the window
     60		 */
     61		m->s[2] = *val;
     62	}
     63	return m->s[0].v;
     64}
     65
     66/* Check if new measurement updates the 1st, 2nd or 3rd choice max. */
     67u32 minmax_running_max(struct minmax *m, u32 win, u32 t, u32 meas)
     68{
     69	struct minmax_sample val = { .t = t, .v = meas };
     70
     71	if (unlikely(val.v >= m->s[0].v) ||	  /* found new max? */
     72	    unlikely(val.t - m->s[2].t > win))	  /* nothing left in window? */
     73		return minmax_reset(m, t, meas);  /* forget earlier samples */
     74
     75	if (unlikely(val.v >= m->s[1].v))
     76		m->s[2] = m->s[1] = val;
     77	else if (unlikely(val.v >= m->s[2].v))
     78		m->s[2] = val;
     79
     80	return minmax_subwin_update(m, win, &val);
     81}
     82EXPORT_SYMBOL(minmax_running_max);
     83
     84/* Check if new measurement updates the 1st, 2nd or 3rd choice min. */
     85u32 minmax_running_min(struct minmax *m, u32 win, u32 t, u32 meas)
     86{
     87	struct minmax_sample val = { .t = t, .v = meas };
     88
     89	if (unlikely(val.v <= m->s[0].v) ||	  /* found new min? */
     90	    unlikely(val.t - m->s[2].t > win))	  /* nothing left in window? */
     91		return minmax_reset(m, t, meas);  /* forget earlier samples */
     92
     93	if (unlikely(val.v <= m->s[1].v))
     94		m->s[2] = m->s[1] = val;
     95	else if (unlikely(val.v <= m->s[2].v))
     96		m->s[2] = val;
     97
     98	return minmax_subwin_update(m, win, &val);
     99}