[Qemu-devel] [RFC PATCH V3 2/3] cpus: add tcg_exec_flag.

[fred . konrad]

From: KONRAD Frederic <address@hidden>

This flag indicates the state of the VCPU thread:
  * 0 if the VCPU is allowed to execute code.
  * 1 if the VCPU is currently executing code.
  * -1 if the VCPU is not allowed to execute code.

This allows to atomically check and run safe work or check and continue the TCG
execution.

Signed-off-by: KONRAD Frederic <address@hidden>
Changes V2 -> V3:
  * introduce a third state which allow or not the execution.
  * atomically check and set the flag when starting or blocking the code 
execution.
Changes V1 -> V2:
  * do both tcg_executing = 0 or 1 in cpu_exec().
---
 cpu-exec.c        |  5 +++++
 include/qom/cpu.h | 32 ++++++++++++++++++++++++++++++++
 qom/cpu.c         | 19 +++++++++++++++++++
 3 files changed, 56 insertions(+)

diff --git a/cpu-exec.c b/cpu-exec.c
index 75694f3..e16666a 100644
--- a/cpu-exec.c
+++ b/cpu-exec.c
@@ -371,6 +371,10 @@ int cpu_exec(CPUState *cpu)
         cpu->halted = 0;
     }
 
+    if (!tcg_cpu_try_start_execution(cpu)) {
+        cpu->exit_request = 1;
+        return 0;
+    }
     current_cpu = cpu;
 
     /* As long as current_cpu is null, up to the assignment just above,
@@ -583,5 +587,6 @@ int cpu_exec(CPUState *cpu)
 
     /* fail safe : never use current_cpu outside cpu_exec() */
     current_cpu = NULL;
+    tcg_cpu_allow_execution(cpu);
     return ret;
 }
diff --git a/include/qom/cpu.h b/include/qom/cpu.h
index efa9624..de7487e 100644
--- a/include/qom/cpu.h
+++ b/include/qom/cpu.h
@@ -226,6 +226,7 @@ struct kvm_run;
  * @stopped: Indicates the CPU has been artificially stopped.
  * @tcg_exit_req: Set to force TCG to stop executing linked TBs for this
  *           CPU and return to its top level loop.
+ * @tcg_exec_flag: See tcg_cpu_flag_* function.
  * @singlestep_enabled: Flags for single-stepping.
  * @icount_extra: Instructions until next timer event.
  * @icount_decr: Number of cycles left, with interrupt flag in high bit.
@@ -322,6 +323,8 @@ struct CPUState {
        (absolute value) offset as small as possible.  This reduces code
        size, especially for hosts without large memory offsets.  */
     volatile sig_atomic_t tcg_exit_req;
+
+    int tcg_exec_flag;
 };
 
 QTAILQ_HEAD(CPUTailQ, CPUState);
@@ -337,6 +340,35 @@ extern struct CPUTailQ cpus;
 DECLARE_TLS(CPUState *, current_cpu);
 #define current_cpu tls_var(current_cpu)
 
+
+/**
+ * tcg_cpu_try_block_execution
+ * @cpu: The CPU to block the execution
+ *
+ * Try to set the tcg_exec_flag to -1 saying the CPU can't execute code if the
+ * CPU is not executing code.
+ * Returns true if the cpu execution is blocked, false otherwise.
+ */
+bool tcg_cpu_try_block_execution(CPUState *cpu);
+
+/**
+ * tcg_cpu_allow_execution
+ * @cpu: The CPU to allow the execution.
+ *
+ * Just reset the state of tcg_exec_flag, and allow the execution of some code.
+ */
+void tcg_cpu_allow_execution(CPUState *cpu);
+
+/**
+ * tcg_cpu_try_start_execution
+ * @cpu: The CPU to start the execution.
+ *
+ * Just set the tcg_exec_flag to 1 saying the CPU is executing code if the CPU
+ * is allowed to run some code.
+ * Returns true if the cpu can execute, false otherwise.
+ */
+bool tcg_cpu_try_start_execution(CPUState *cpu);
+
 /**
  * cpu_paging_enabled:
  * @cpu: The CPU whose state is to be inspected.
diff --git a/qom/cpu.c b/qom/cpu.c
index 4e12598..e32f90c 100644
--- a/qom/cpu.c
+++ b/qom/cpu.c
@@ -26,6 +26,23 @@
 #include "qemu/error-report.h"
 #include "sysemu/sysemu.h"
 
+bool tcg_cpu_try_block_execution(CPUState *cpu)
+{
+    return (atomic_cmpxchg(&cpu->tcg_exec_flag, 0, -1)
+           || (cpu->tcg_exec_flag == -1));
+}
+
+void tcg_cpu_allow_execution(CPUState *cpu)
+{
+    cpu->tcg_exec_flag = 0;
+}
+
+bool tcg_cpu_try_start_execution(CPUState *cpu)
+{
+    return (atomic_cmpxchg(&cpu->tcg_exec_flag, 0, 1)
+           || (cpu->tcg_exec_flag == 1));
+}
+
 bool cpu_exists(int64_t id)
 {
     CPUState *cpu;
@@ -249,6 +266,8 @@ static void cpu_common_reset(CPUState *cpu)
     cpu->icount_decr.u32 = 0;
     cpu->can_do_io = 0;
     cpu->exception_index = -1;
+
+    tcg_cpu_allow_execution(cpu);
     memset(cpu->tb_jmp_cache, 0, TB_JMP_CACHE_SIZE * sizeof(void *));
 }
 
-- 
1.9.0