/* | |

* Hierarchical Bitmap Data Type | |

* | |

* Copyright Red Hat, Inc., 2012 | |

* | |

* Author: Paolo Bonzini <pbonzini@redhat.com> | |

* | |

* This work is licensed under the terms of the GNU GPL, version 2 or | |

* later. See the COPYING file in the top-level directory. | |

*/ | |

#include <string.h> | |

#include <glib.h> | |

#include <assert.h> | |

#include "qemu/osdep.h" | |

#include "qemu/hbitmap.h" | |

#include "qemu/host-utils.h" | |

#include "trace.h" | |

/* HBitmaps provides an array of bits. The bits are stored as usual in an | |

* array of unsigned longs, but HBitmap is also optimized to provide fast | |

* iteration over set bits; going from one bit to the next is O(logB n) | |

* worst case, with B = sizeof(long) * CHAR_BIT: the result is low enough | |

* that the number of levels is in fact fixed. | |

* | |

* In order to do this, it stacks multiple bitmaps with progressively coarser | |

* granularity; in all levels except the last, bit N is set iff the N-th | |

* unsigned long is nonzero in the immediately next level. When iteration | |

* completes on the last level it can examine the 2nd-last level to quickly | |

* skip entire words, and even do so recursively to skip blocks of 64 words or | |

* powers thereof (32 on 32-bit machines). | |

* | |

* Given an index in the bitmap, it can be split in group of bits like | |

* this (for the 64-bit case): | |

* | |

* bits 0-57 => word in the last bitmap | bits 58-63 => bit in the word | |

* bits 0-51 => word in the 2nd-last bitmap | bits 52-57 => bit in the word | |

* bits 0-45 => word in the 3rd-last bitmap | bits 46-51 => bit in the word | |

* | |

* So it is easy to move up simply by shifting the index right by | |

* log2(BITS_PER_LONG) bits. To move down, you shift the index left | |

* similarly, and add the word index within the group. Iteration uses | |

* ffs (find first set bit) to find the next word to examine; this | |

* operation can be done in constant time in most current architectures. | |

* | |

* Setting or clearing a range of m bits on all levels, the work to perform | |

* is O(m + m/W + m/W^2 + ...), which is O(m) like on a regular bitmap. | |

* | |

* When iterating on a bitmap, each bit (on any level) is only visited | |

* once. Hence, The total cost of visiting a bitmap with m bits in it is | |

* the number of bits that are set in all bitmaps. Unless the bitmap is | |

* extremely sparse, this is also O(m + m/W + m/W^2 + ...), so the amortized | |

* cost of advancing from one bit to the next is usually constant (worst case | |

* O(logB n) as in the non-amortized complexity). | |

*/ | |

struct HBitmap { | |

/* Number of total bits in the bottom level. */ | |

uint64_t size; | |

/* Number of set bits in the bottom level. */ | |

uint64_t count; | |

/* A scaling factor. Given a granularity of G, each bit in the bitmap will | |

* will actually represent a group of 2^G elements. Each operation on a | |

* range of bits first rounds the bits to determine which group they land | |

* in, and then affect the entire page; iteration will only visit the first | |

* bit of each group. Here is an example of operations in a size-16, | |

* granularity-1 HBitmap: | |

* | |

* initial state 00000000 | |

* set(start=0, count=9) 11111000 (iter: 0, 2, 4, 6, 8) | |

* reset(start=1, count=3) 00111000 (iter: 4, 6, 8) | |

* set(start=9, count=2) 00111100 (iter: 4, 6, 8, 10) | |

* reset(start=5, count=5) 00000000 | |

* | |

* From an implementation point of view, when setting or resetting bits, | |

* the bitmap will scale bit numbers right by this amount of bits. When | |

* iterating, the bitmap will scale bit numbers left by this amount of | |

* bits. | |

*/ | |

int granularity; | |

/* A number of progressively less coarse bitmaps (i.e. level 0 is the | |

* coarsest). Each bit in level N represents a word in level N+1 that | |

* has a set bit, except the last level where each bit represents the | |

* actual bitmap. | |

* | |

* Note that all bitmaps have the same number of levels. Even a 1-bit | |

* bitmap will still allocate HBITMAP_LEVELS arrays. | |

*/ | |

unsigned long *levels[HBITMAP_LEVELS]; | |

}; | |

/* Advance hbi to the next nonzero word and return it. hbi->pos | |

* is updated. Returns zero if we reach the end of the bitmap. | |

*/ | |

unsigned long hbitmap_iter_skip_words(HBitmapIter *hbi) | |

{ | |

size_t pos = hbi->pos; | |

const HBitmap *hb = hbi->hb; | |

unsigned i = HBITMAP_LEVELS - 1; | |

unsigned long cur; | |

do { | |

cur = hbi->cur[--i]; | |

pos >>= BITS_PER_LEVEL; | |

} while (cur == 0); | |

/* Check for end of iteration. We always use fewer than BITS_PER_LONG | |

* bits in the level 0 bitmap; thus we can repurpose the most significant | |

* bit as a sentinel. The sentinel is set in hbitmap_alloc and ensures | |

* that the above loop ends even without an explicit check on i. | |

*/ | |

if (i == 0 && cur == (1UL << (BITS_PER_LONG - 1))) { | |

return 0; | |

} | |

for (; i < HBITMAP_LEVELS - 1; i++) { | |

/* Shift back pos to the left, matching the right shifts above. | |

* The index of this word's least significant set bit provides | |

* the low-order bits. | |

*/ | |

assert(cur); | |

pos = (pos << BITS_PER_LEVEL) + ctzl(cur); | |

hbi->cur[i] = cur & (cur - 1); | |

/* Set up next level for iteration. */ | |

cur = hb->levels[i + 1][pos]; | |

} | |

hbi->pos = pos; | |

trace_hbitmap_iter_skip_words(hbi->hb, hbi, pos, cur); | |

assert(cur); | |

return cur; | |

} | |

void hbitmap_iter_init(HBitmapIter *hbi, const HBitmap *hb, uint64_t first) | |

{ | |

unsigned i, bit; | |

uint64_t pos; | |

hbi->hb = hb; | |

pos = first >> hb->granularity; | |

assert(pos < hb->size); | |

hbi->pos = pos >> BITS_PER_LEVEL; | |

hbi->granularity = hb->granularity; | |

for (i = HBITMAP_LEVELS; i-- > 0; ) { | |

bit = pos & (BITS_PER_LONG - 1); | |

pos >>= BITS_PER_LEVEL; | |

/* Drop bits representing items before first. */ | |

hbi->cur[i] = hb->levels[i][pos] & ~((1UL << bit) - 1); | |

/* We have already added level i+1, so the lowest set bit has | |

* been processed. Clear it. | |

*/ | |

if (i != HBITMAP_LEVELS - 1) { | |

hbi->cur[i] &= ~(1UL << bit); | |

} | |

} | |

} | |

bool hbitmap_empty(const HBitmap *hb) | |

{ | |

return hb->count == 0; | |

} | |

int hbitmap_granularity(const HBitmap *hb) | |

{ | |

return hb->granularity; | |

} | |

uint64_t hbitmap_count(const HBitmap *hb) | |

{ | |

return hb->count << hb->granularity; | |

} | |

/* Count the number of set bits between start and end, not accounting for | |

* the granularity. Also an example of how to use hbitmap_iter_next_word. | |

*/ | |

static uint64_t hb_count_between(HBitmap *hb, uint64_t start, uint64_t last) | |

{ | |

HBitmapIter hbi; | |

uint64_t count = 0; | |

uint64_t end = last + 1; | |

unsigned long cur; | |

size_t pos; | |

hbitmap_iter_init(&hbi, hb, start << hb->granularity); | |

for (;;) { | |

pos = hbitmap_iter_next_word(&hbi, &cur); | |

if (pos >= (end >> BITS_PER_LEVEL)) { | |

break; | |

} | |

count += ctpopl(cur); | |

} | |

if (pos == (end >> BITS_PER_LEVEL)) { | |

/* Drop bits representing the END-th and subsequent items. */ | |

int bit = end & (BITS_PER_LONG - 1); | |

cur &= (1UL << bit) - 1; | |

count += ctpopl(cur); | |

} | |

return count; | |

} | |

/* Setting starts at the last layer and propagates up if an element | |

* changes from zero to non-zero. | |

*/ | |

static inline bool hb_set_elem(unsigned long *elem, uint64_t start, uint64_t last) | |

{ | |

unsigned long mask; | |

bool changed; | |

assert((last >> BITS_PER_LEVEL) == (start >> BITS_PER_LEVEL)); | |

assert(start <= last); | |

mask = 2UL << (last & (BITS_PER_LONG - 1)); | |

mask -= 1UL << (start & (BITS_PER_LONG - 1)); | |

changed = (*elem == 0); | |

*elem |= mask; | |

return changed; | |

} | |

/* The recursive workhorse (the depth is limited to HBITMAP_LEVELS)... */ | |

static void hb_set_between(HBitmap *hb, int level, uint64_t start, uint64_t last) | |

{ | |

size_t pos = start >> BITS_PER_LEVEL; | |

size_t lastpos = last >> BITS_PER_LEVEL; | |

bool changed = false; | |

size_t i; | |

i = pos; | |

if (i < lastpos) { | |

uint64_t next = (start | (BITS_PER_LONG - 1)) + 1; | |

changed |= hb_set_elem(&hb->levels[level][i], start, next - 1); | |

for (;;) { | |

start = next; | |

next += BITS_PER_LONG; | |

if (++i == lastpos) { | |

break; | |

} | |

changed |= (hb->levels[level][i] == 0); | |

hb->levels[level][i] = ~0UL; | |

} | |

} | |

changed |= hb_set_elem(&hb->levels[level][i], start, last); | |

/* If there was any change in this layer, we may have to update | |

* the one above. | |

*/ | |

if (level > 0 && changed) { | |

hb_set_between(hb, level - 1, pos, lastpos); | |

} | |

} | |

void hbitmap_set(HBitmap *hb, uint64_t start, uint64_t count) | |

{ | |

/* Compute range in the last layer. */ | |

uint64_t last = start + count - 1; | |

trace_hbitmap_set(hb, start, count, | |

start >> hb->granularity, last >> hb->granularity); | |

start >>= hb->granularity; | |

last >>= hb->granularity; | |

count = last - start + 1; | |

hb->count += count - hb_count_between(hb, start, last); | |

hb_set_between(hb, HBITMAP_LEVELS - 1, start, last); | |

} | |

/* Resetting works the other way round: propagate up if the new | |

* value is zero. | |

*/ | |

static inline bool hb_reset_elem(unsigned long *elem, uint64_t start, uint64_t last) | |

{ | |

unsigned long mask; | |

bool blanked; | |

assert((last >> BITS_PER_LEVEL) == (start >> BITS_PER_LEVEL)); | |

assert(start <= last); | |

mask = 2UL << (last & (BITS_PER_LONG - 1)); | |

mask -= 1UL << (start & (BITS_PER_LONG - 1)); | |

blanked = *elem != 0 && ((*elem & ~mask) == 0); | |

*elem &= ~mask; | |

return blanked; | |

} | |

/* The recursive workhorse (the depth is limited to HBITMAP_LEVELS)... */ | |

static void hb_reset_between(HBitmap *hb, int level, uint64_t start, uint64_t last) | |

{ | |

size_t pos = start >> BITS_PER_LEVEL; | |

size_t lastpos = last >> BITS_PER_LEVEL; | |

bool changed = false; | |

size_t i; | |

i = pos; | |

if (i < lastpos) { | |

uint64_t next = (start | (BITS_PER_LONG - 1)) + 1; | |

/* Here we need a more complex test than when setting bits. Even if | |

* something was changed, we must not blank bits in the upper level | |

* unless the lower-level word became entirely zero. So, remove pos | |

* from the upper-level range if bits remain set. | |

*/ | |

if (hb_reset_elem(&hb->levels[level][i], start, next - 1)) { | |

changed = true; | |

} else { | |

pos++; | |

} | |

for (;;) { | |

start = next; | |

next += BITS_PER_LONG; | |

if (++i == lastpos) { | |

break; | |

} | |

changed |= (hb->levels[level][i] != 0); | |

hb->levels[level][i] = 0UL; | |

} | |

} | |

/* Same as above, this time for lastpos. */ | |

if (hb_reset_elem(&hb->levels[level][i], start, last)) { | |

changed = true; | |

} else { | |

lastpos--; | |

} | |

if (level > 0 && changed) { | |

hb_reset_between(hb, level - 1, pos, lastpos); | |

} | |

} | |

void hbitmap_reset(HBitmap *hb, uint64_t start, uint64_t count) | |

{ | |

/* Compute range in the last layer. */ | |

uint64_t last = start + count - 1; | |

trace_hbitmap_reset(hb, start, count, | |

start >> hb->granularity, last >> hb->granularity); | |

start >>= hb->granularity; | |

last >>= hb->granularity; | |

hb->count -= hb_count_between(hb, start, last); | |

hb_reset_between(hb, HBITMAP_LEVELS - 1, start, last); | |

} | |

bool hbitmap_get(const HBitmap *hb, uint64_t item) | |

{ | |

/* Compute position and bit in the last layer. */ | |

uint64_t pos = item >> hb->granularity; | |

unsigned long bit = 1UL << (pos & (BITS_PER_LONG - 1)); | |

return (hb->levels[HBITMAP_LEVELS - 1][pos >> BITS_PER_LEVEL] & bit) != 0; | |

} | |

void hbitmap_free(HBitmap *hb) | |

{ | |

unsigned i; | |

for (i = HBITMAP_LEVELS; i-- > 0; ) { | |

g_free(hb->levels[i]); | |

} | |

g_free(hb); | |

} | |

HBitmap *hbitmap_alloc(uint64_t size, int granularity) | |

{ | |

HBitmap *hb = g_malloc0(sizeof (struct HBitmap)); | |

unsigned i; | |

assert(granularity >= 0 && granularity < 64); | |

size = (size + (1ULL << granularity) - 1) >> granularity; | |

assert(size <= ((uint64_t)1 << HBITMAP_LOG_MAX_SIZE)); | |

hb->size = size; | |

hb->granularity = granularity; | |

for (i = HBITMAP_LEVELS; i-- > 0; ) { | |

size = MAX((size + BITS_PER_LONG - 1) >> BITS_PER_LEVEL, 1); | |

hb->levels[i] = g_malloc0(size * sizeof(unsigned long)); | |

} | |

/* We necessarily have free bits in level 0 due to the definition | |

* of HBITMAP_LEVELS, so use one for a sentinel. This speeds up | |

* hbitmap_iter_skip_words. | |

*/ | |

assert(size == 1); | |

hb->levels[0][0] |= 1UL << (BITS_PER_LONG - 1); | |

return hb; | |

} |