[Qemu-devel] [PATCH v2 1/6] tcg: Ensure safe tb_jmp_cache lookup out of 'tb_lock'

["Alex Bennée" <alex.bennee@linaro.org>]

From: Sergey Fedorov <serge.fdrv@gmail.com>

First, ensure atomicity of CPU's 'tb_jmp_cache' access by:
 * using atomic_read() to look up a TB when not holding 'tb_lock';
 * using atomic_write() to remove a TB from each CPU's local cache on
   TB invalidation.

Second, add some memory barriers to ensure we don't put the TB being
invalidated back to CPU's 'tb_jmp_cache'. If we fail to look up a TB in
CPU's local cache because it is being invalidated by some other thread
then it must not be found in the shared TB hash table. Otherwise we'd
put it back to CPU's local cache.

Note that this patch does *not* make CPU's TLB invalidation safe if it
is done from some other thread while the CPU is in its execution loop.

Signed-off-by: Sergey Fedorov <serge.fdrv@gmail.com>
Signed-off-by: Sergey Fedorov <sergey.fedorov@linaro.org>
[AJB: fixed missing atomic set, tweak title]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Reviewed-by: Emilio G. Cota <cota@braap.org>

---
v1 (AJB):
  - tweak title
  - fixed missing set of tb_jmp_cache
v2
  - fix spelling s/con't/can't/
  - add atomic_read while clearing tb_jmp_cache
  - add r-b tags
---
 cpu-exec.c      | 9 +++++++--
 translate-all.c | 9 +++++++--
 2 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/cpu-exec.c b/cpu-exec.c
index b840e1d..10ce1cb 100644
--- a/cpu-exec.c
+++ b/cpu-exec.c
@@ -285,6 +285,11 @@ static TranslationBlock *tb_find_slow(CPUState *cpu,
 {
     TranslationBlock *tb;
 
+    /* Ensure that we won't find a TB in the shared hash table
+     * if it is being invalidated by some other thread.
+     * Otherwise we'd put it back to CPU's local cache.
+     * Pairs with smp_wmb() in tb_phys_invalidate(). */
+    smp_rmb();
     tb = tb_find_physical(cpu, pc, cs_base, flags);
     if (tb) {
         goto found;
@@ -315,7 +320,7 @@ static TranslationBlock *tb_find_slow(CPUState *cpu,
 
 found:
     /* we add the TB in the virtual pc hash table */
-    cpu->tb_jmp_cache[tb_jmp_cache_hash_func(pc)] = tb;
+    atomic_set(&cpu->tb_jmp_cache[tb_jmp_cache_hash_func(pc)], tb);
     return tb;
 }
 
@@ -333,7 +338,7 @@ static inline TranslationBlock *tb_find_fast(CPUState *cpu,
        is executed. */
     cpu_get_tb_cpu_state(env, &pc, &cs_base, &flags);
     tb_lock();
-    tb = cpu->tb_jmp_cache[tb_jmp_cache_hash_func(pc)];
+    tb = atomic_read(&cpu->tb_jmp_cache[tb_jmp_cache_hash_func(pc)]);
     if (unlikely(!tb || tb->pc != pc || tb->cs_base != cs_base ||
                  tb->flags != flags)) {
         tb = tb_find_slow(cpu, pc, cs_base, flags);
diff --git a/translate-all.c b/translate-all.c
index eaa95e4..96efe48 100644
--- a/translate-all.c
+++ b/translate-all.c
@@ -1004,11 +1004,16 @@ void tb_phys_invalidate(TranslationBlock *tb, tb_page_addr_t page_addr)
         invalidate_page_bitmap(p);
     }
 
+    /* Ensure that we won't find the TB in the shared hash table
+     * if we can't see it in CPU's local cache.
+     * Pairs with smp_rmb() in tb_find_slow(). */
+    smp_wmb();
+
     /* remove the TB from the hash list */
     h = tb_jmp_cache_hash_func(tb->pc);
     CPU_FOREACH(cpu) {
-        if (cpu->tb_jmp_cache[h] == tb) {
-            cpu->tb_jmp_cache[h] = NULL;
+        if (atomic_read(&cpu->tb_jmp_cache[h]) == tb) {
+            atomic_set(&cpu->tb_jmp_cache[h], NULL);
         }
     }
 
-- 
2.7.4