tcg: consolidate TB lookups in tb_lookup__cpu_state
This avoids duplicating code. cpu_exec_step will also use the
new common function once we integrate parallel_cpus into tb->cflags.
Note that in this commit we also fix a race, described by Richard Henderson
during review. Think of this scenario with threads A and B:
(A) Lookup succeeds for TB in hash without tb_lock
(B) Sets the TB's tb->invalid flag
(B) Removes the TB from tb_htable
(B) Clears all CPU's tb_jmp_cache
(A) Store TB into local tb_jmp_cache
Given that order of events, (A) will keep executing that invalid TB until
another flush of its tb_jmp_cache happens, which in theory might never happen.
We can fix this by checking the tb->invalid flag every time we look up a TB
from tb_jmp_cache, so that in the above scenario, next time we try to find
that TB in tb_jmp_cache, we won't, and will therefore be forced to look it
up in tb_htable.
Performance-wise, I measured a small improvement when booting debian-arm.
Note that inlining pays off:
Performance counter stats for 'taskset -c 0 qemu-system-arm \
-machine type=virt -nographic -smp 1 -m 4096 \
-netdev user,id=unet,hostfwd=tcp::2222-:22 \
-device virtio-net-device,netdev=unet \
-drive file=jessie.qcow2,id=myblock,index=0,if=none \
-device virtio-blk-device,drive=myblock \
-kernel kernel.img -append console=ttyAMA0 root=/dev/vda1 \
-name arm,debug-threads=on -smp 1' (10 runs):
Before:
18714.917392 task-clock # 0.952 CPUs utilized ( +- 0.95% )
23,142 context-switches # 0.001 M/sec ( +- 0.50% )
1 CPU-migrations # 0.000 M/sec
10,558 page-faults # 0.001 M/sec ( +- 0.95% )
53,957,727,252 cycles # 2.883 GHz ( +- 0.91% ) [83.33%]
24,440,599,852 stalled-cycles-frontend # 45.30% frontend cycles idle ( +- 1.20% ) [83.33%]
16,495,714,424 stalled-cycles-backend # 30.57% backend cycles idle ( +- 0.95% ) [66.66%]
76,267,572,582 instructions # 1.41 insns per cycle
# 0.32 stalled cycles per insn ( +- 0.87% ) [83.34%]
12,692,186,323 branches # 678.186 M/sec ( +- 0.92% ) [83.35%]
263,486,879 branch-misses # 2.08% of all branches ( +- 0.73% ) [83.34%]
19.648474449 seconds time elapsed ( +- 0.82% )
After, w/ inline (this patch):
18471.376627 task-clock # 0.955 CPUs utilized ( +- 0.96% )
23,048 context-switches # 0.001 M/sec ( +- 0.48% )
1 CPU-migrations # 0.000 M/sec
10,708 page-faults # 0.001 M/sec ( +- 0.81% )
53,208,990,796 cycles # 2.881 GHz ( +- 0.98% ) [83.34%]
23,941,071,673 stalled-cycles-frontend # 44.99% frontend cycles idle ( +- 0.95% ) [83.34%]
16,161,773,848 stalled-cycles-backend # 30.37% backend cycles idle ( +- 0.76% ) [66.67%]
75,786,269,766 instructions # 1.42 insns per cycle
# 0.32 stalled cycles per insn ( +- 1.24% ) [83.34%]
12,573,617,143 branches # 680.708 M/sec ( +- 1.34% ) [83.33%]
260,235,550 branch-misses # 2.07% of all branches ( +- 0.66% ) [83.33%]
19.340502161 seconds time elapsed ( +- 0.56% )
After, w/o inline:
18791.253967 task-clock # 0.954 CPUs utilized ( +- 0.78% )
23,230 context-switches # 0.001 M/sec ( +- 0.42% )
1 CPU-migrations # 0.000 M/sec
10,563 page-faults # 0.001 M/sec ( +- 1.27% )
54,168,674,622 cycles # 2.883 GHz ( +- 0.80% ) [83.34%]
24,244,712,629 stalled-cycles-frontend # 44.76% frontend cycles idle ( +- 1.37% ) [83.33%]
16,288,648,572 stalled-cycles-backend # 30.07% backend cycles idle ( +- 0.95% ) [66.66%]
77,659,755,503 instructions # 1.43 insns per cycle
# 0.31 stalled cycles per insn ( +- 0.97% ) [83.34%]
12,922,780,045 branches # 687.702 M/sec ( +- 1.06% ) [83.34%]
261,962,386 branch-misses # 2.03% of all branches ( +- 0.71% ) [83.35%]
19.700174670 seconds time elapsed ( +- 0.56% )
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
diff --git a/accel/tcg/cpu-exec.c b/accel/tcg/cpu-exec.c
index 32104b8..f8a1d68 100644
--- a/accel/tcg/cpu-exec.c
+++ b/accel/tcg/cpu-exec.c
@@ -28,6 +28,7 @@
#include "exec/address-spaces.h"
#include "qemu/rcu.h"
#include "exec/tb-hash.h"
+#include "exec/tb-lookup.h"
#include "exec/log.h"
#include "qemu/main-loop.h"
#if defined(TARGET_I386) && !defined(CONFIG_USER_ONLY)
@@ -368,43 +369,31 @@
TranslationBlock *last_tb,
int tb_exit)
{
- CPUArchState *env = (CPUArchState *)cpu->env_ptr;
TranslationBlock *tb;
target_ulong cs_base, pc;
uint32_t flags;
bool acquired_tb_lock = false;
- /* we record a subset of the CPU state. It will
- always be the same before a given translated block
- is executed. */
- cpu_get_tb_cpu_state(env, &pc, &cs_base, &flags);
- tb = atomic_rcu_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->trace_vcpu_dstate != *cpu->trace_dstate)) {
+ tb = tb_lookup__cpu_state(cpu, &pc, &cs_base, &flags);
+ if (tb == NULL) {
+ /* mmap_lock is needed by tb_gen_code, and mmap_lock must be
+ * taken outside tb_lock. As system emulation is currently
+ * single threaded the locks are NOPs.
+ */
+ mmap_lock();
+ tb_lock();
+ acquired_tb_lock = true;
+
+ /* There's a chance that our desired tb has been translated while
+ * taking the locks so we check again inside the lock.
+ */
tb = tb_htable_lookup(cpu, pc, cs_base, flags);
- if (!tb) {
-
- /* mmap_lock is needed by tb_gen_code, and mmap_lock must be
- * taken outside tb_lock. As system emulation is currently
- * single threaded the locks are NOPs.
- */
- mmap_lock();
- tb_lock();
- acquired_tb_lock = true;
-
- /* There's a chance that our desired tb has been translated while
- * taking the locks so we check again inside the lock.
- */
- tb = tb_htable_lookup(cpu, pc, cs_base, flags);
- if (!tb) {
- /* if no translated code available, then translate it now */
- tb = tb_gen_code(cpu, pc, cs_base, flags, 0);
- }
-
- mmap_unlock();
+ if (likely(tb == NULL)) {
+ /* if no translated code available, then translate it now */
+ tb = tb_gen_code(cpu, pc, cs_base, flags, 0);
}
+ mmap_unlock();
/* We add the TB in the virtual pc hash table for the fast lookup */
atomic_set(&cpu->tb_jmp_cache[tb_jmp_cache_hash_func(pc)], tb);
}
