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exec: optimize phys_page_set_level 

phys_page_set_level is writing zeroes to a struct that has just been
filled in by phys_map_node_alloc.  Instead, tell phys_map_node_alloc
whether to fill in the page "as a leaf" or "as a non-leaf".

memcpy is faster than struct assignment, which copies each bitfield
individually.  A compiler bug (https://gcc.gnu.org/PR66391), and
small memcpys like this one are special-cased anyway, and optimized
to a register move, so just use the memcpy.

This cuts the cost of phys_page_set_level from 25% to 5% when
booting qboot.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Paolo Bonzini 2015-05-21 15:12:29 +02:00
parent e4afbf4fb4
commit db94604b20
1 changed files with 10 additions and 14 deletions
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24
exec.c
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@ -173,17 +173,22 @@ static void phys_map_node_reserve(PhysPageMap *map, unsigned nodes)
}
}
static uint32_t phys_map_node_alloc(PhysPageMap *map)
static uint32_t phys_map_node_alloc(PhysPageMap *map, bool leaf)
{
unsigned i;
uint32_t ret;
PhysPageEntry e;
PhysPageEntry *p;
ret = map->nodes_nb++;
p = map->nodes[ret];
assert(ret != PHYS_MAP_NODE_NIL);
assert(ret != map->nodes_nb_alloc);
e.skip = leaf ? 0 : 1;
e.ptr = leaf ? PHYS_SECTION_UNASSIGNED : PHYS_MAP_NODE_NIL;
for (i = 0; i < P_L2_SIZE; ++i) {
map->nodes[ret][i].skip = 1;
map->nodes[ret][i].ptr = PHYS_MAP_NODE_NIL;
memcpy(&p[i], &e, sizeof(e));
}
return ret;
}
@ -193,21 +198,12 @@ static void phys_page_set_level(PhysPageMap *map, PhysPageEntry *lp,
int level)
{
PhysPageEntry *p;
int i;
hwaddr step = (hwaddr)1 << (level * P_L2_BITS);
if (lp->skip && lp->ptr == PHYS_MAP_NODE_NIL) {
lp->ptr = phys_map_node_alloc(map);
p = map->nodes[lp->ptr];
if (level == 0) {
for (i = 0; i < P_L2_SIZE; i++) {
p[i].skip = 0;
p[i].ptr = PHYS_SECTION_UNASSIGNED;
}
}
} else {
p = map->nodes[lp->ptr];
lp->ptr = phys_map_node_alloc(map, level == 0);
}
p = map->nodes[lp->ptr];
lp = &p[(*index >> (level * P_L2_BITS)) & (P_L2_SIZE - 1)];
while (*nb && lp < &p[P_L2_SIZE]) {