Re: [Qemu-devel] [PATCH] mttcg: Set jmp_env to handle exit from tb_gen_code

[Pranith Kumar <bobby.prani@gmail.com>]

Alex Bennée writes:

> Pranith Kumar <bobby.prani@gmail.com> writes:
>
>> tb_gen_code() can exit execution using cpu_exit_loop() when it cannot
>> allocate new tb's. To handle this, we need to properly set the jmp_env
>> pointer ahead of calling tb_gen_code().
>>
>> CC:Alex Bennée <alex.bennee@linaro.org>
>> CC: Richard Henderson <rth@twiddle.net>
>> Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
>> ---
>>  cpu-exec.c | 23 +++++++++++------------
>>  1 file changed, 11 insertions(+), 12 deletions(-)
>>
>> diff --git a/cpu-exec.c b/cpu-exec.c
>> index 97d79612d9..4b70988b24 100644
>> --- a/cpu-exec.c
>> +++ b/cpu-exec.c
>> @@ -236,23 +236,22 @@ static void cpu_exec_step(CPUState *cpu)
>>
>>      cpu_get_tb_cpu_state(env, &pc, &cs_base, &flags);
>>      tb_lock();
>> -    tb = tb_gen_code(cpu, pc, cs_base, flags,
>> -                     1 | CF_NOCACHE | CF_IGNORE_ICOUNT);
>> -    tb->orig_tb = NULL;
>> -    tb_unlock();
>> -
>> -    cc->cpu_exec_enter(cpu);
>> -
>
> It occurs to me we are also diverging in our locking pattern from
> tb_find which takes mmap_lock first. This is a NOP for system emulation
> but needed for user-emulation (for which we can do cpu_exec_step but not
> cpu_exec_nocache).

Right. So we have to take the mmap_lock() before calling
tb_gen_code(). However, this lock is released in the error path before calling
cpu_loop_exit() if allocation of a new tb fails. The following is what I have
after merging with the previous EXCP_ATOMIC handling patch.

diff --git a/cpu-exec.c b/cpu-exec.c
index a8e04bffbf..2bb3ba3672 100644
--- a/cpu-exec.c
+++ b/cpu-exec.c
@@ -228,6 +228,7 @@ static void cpu_exec_nocache(CPUState *cpu, int max_cycles,
 
 static void cpu_exec_step(CPUState *cpu)
 {
+    CPUClass *cc = CPU_GET_CLASS(cpu);
     CPUArchState *env = (CPUArchState *)cpu->env_ptr;
     TranslationBlock *tb;
     target_ulong cs_base, pc;
@@ -235,16 +236,24 @@ static void cpu_exec_step(CPUState *cpu)
 
     cpu_get_tb_cpu_state(env, &pc, &cs_base, &flags);
     tb_lock();
-    tb = tb_gen_code(cpu, pc, cs_base, flags,
-                     1 | CF_NOCACHE | CF_IGNORE_ICOUNT);
-    tb->orig_tb = NULL;
-    tb_unlock();
-    /* execute the generated code */
-    trace_exec_tb_nocache(tb, pc);
-    cpu_tb_exec(cpu, tb);
-    tb_lock();
-    tb_phys_invalidate(tb, -1);
-    tb_free(tb);
+    if (sigsetjmp(cpu->jmp_env, 0) == 0) {
+        mmap_lock();
+        tb = tb_gen_code(cpu, pc, cs_base, flags,
+                         1 | CF_NOCACHE | CF_IGNORE_ICOUNT);
+        tb->orig_tb = NULL;
+        mmap_unlock();
+        tb_unlock();
+
+        cc->cpu_exec_enter(cpu);
+        /* execute the generated code */
+        trace_exec_tb_nocache(tb, pc);
+        cpu_tb_exec(cpu, tb);
+        cc->cpu_exec_exit(cpu);
+
+        tb_lock();
+        tb_phys_invalidate(tb, -1);
+        tb_free(tb);
+    }
     tb_unlock();
 }
 
diff --git a/cpus.c b/cpus.c
index 77bba08f9a..b39408b4b1 100644
--- a/cpus.c
+++ b/cpus.c
@@ -1347,6 +1347,11 @@ static void *qemu_tcg_rr_cpu_thread_fn(void *arg)
                 if (r == EXCP_DEBUG) {
                     cpu_handle_guest_debug(cpu);
                     break;
+                } else if (r == EXCP_ATOMIC) {
+                    qemu_mutex_unlock_iothread();
+                    cpu_exec_step_atomic(cpu);
+                    qemu_mutex_lock_iothread();
+                    break;
                 }
             } else if (cpu->stop) {
                 if (cpu->unplug) {
@@ -1457,6 +1462,10 @@ static void *qemu_tcg_cpu_thread_fn(void *arg)
                  */
                 g_assert(cpu->halted);
                 break;
+            case EXCP_ATOMIC:
+                qemu_mutex_unlock_iothread();
+                cpu_exec_step_atomic(cpu);
+                qemu_mutex_lock_iothread();
             default:
                 /* Ignore everything else? */
                 break;


--
Pranith