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translate-all: make l1_map lockless 

Groundwork for supporting parallel TCG generation.

We never remove entries from the radix tree, so we can use cmpxchg
to implement lockless insertions.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
This commit is contained in:
Emilio G. Cota 2017-07-26 20:15:41 -04:00 committed by Richard Henderson
parent 1e05197f24
commit 78722ed0b8
2 changed files with 16 additions and 12 deletions
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24
accel/tcg/translate-all.c
View File

@ -469,20 +469,12 @@ static void page_init(void)
#endif
}
/* If alloc=1:
* Called with tb_lock held for system emulation.
* Called with mmap_lock held for user-mode emulation.
*/
static PageDesc *page_find_alloc(tb_page_addr_t index, int alloc)
{
PageDesc *pd;
void **lp;
int i;
if (alloc) {
assert_memory_lock();
}
/* Level 1. Always allocated. */
lp = l1_map + ((index >> v_l1_shift) & (v_l1_size - 1));
@ -491,11 +483,17 @@ static PageDesc *page_find_alloc(tb_page_addr_t index, int alloc)
void **p = atomic_rcu_read(lp);
if (p == NULL) {
void *existing;
if (!alloc) {
return NULL;
}
p = g_new0(void *, V_L2_SIZE);
atomic_rcu_set(lp, p);
existing = atomic_cmpxchg(lp, NULL, p);
if (unlikely(existing)) {
g_free(p);
p = existing;
}
}
lp = p + ((index >> (i * V_L2_BITS)) & (V_L2_SIZE - 1));
@ -503,11 +501,17 @@ static PageDesc *page_find_alloc(tb_page_addr_t index, int alloc)
pd = atomic_rcu_read(lp);
if (pd == NULL) {
void *existing;
if (!alloc) {
return NULL;
}
pd = g_new0(PageDesc, V_L2_SIZE);
atomic_rcu_set(lp, pd);
existing = atomic_cmpxchg(lp, NULL, pd);
if (unlikely(existing)) {
g_free(pd);
pd = existing;
}
}
return pd + (index & (V_L2_SIZE - 1));

4
docs/devel/multi-thread-tcg.txt
View File

@ -134,8 +134,8 @@ tb_set_jmp_target() code. Modification to the linked lists that allow
searching for linked pages are done under the protect of the
tb_lock().
The global page table is protected by the tb_lock() in system-mode and
mmap_lock() in linux-user mode.
The global page table is a lockless radix tree; cmpxchg is used
to atomically insert new elements.
The lookup caches are updated atomically and the lookup hash uses QHT
which is designed for concurrent safe lookup.