[v2,5/7] accel/tcg: Change BQL critical section in cpu_handle_interrupt

Commit Message

Robert Foley Aug. 19, 2020, 6:28 p.m. UTC

We are changing the critical section from being around
the majority of the cpu_handle_interrupt to instead
be around just the call to ->cpu_exec_interrupt.

This is in preparation for pushing down the BQL into the
per arch implementation.

We should mention that we discussed these changes as well as
some open questions here:
https://lists.gnu.org/archive/html/qemu-devel/2020-08/msg01189.html
With the main open question being:
BQL is clearly needed to protect the critical section around the call to
->cpu_exec_interrupt.  What else is the BQL protecting in cpu_handle_interrupt
that we need to consider?  Are we missing anything here?

Signed-off-by: Robert Foley <[e-mail]>
---
 accel/tcg/cpu-exec.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

Comments

Richard Henderson Aug. 31, 2020, 9:44 p.m. UTC | #1

On 8/19/20 11:28 AM, Robert Foley wrote:
> @@ -625,7 +624,6 @@ static inline bool cpu_handle_interrupt(CPUState *cpu,
>          }
>  
>          /* If we exit via cpu_loop_exit/longjmp it is reset in cpu_exec */
> -        qemu_mutex_unlock_iothread();

The comment no longer has an "it" to reference.
We should probably expand that to mention which lock.

Otherwise,
Reviewed-by: Richard Henderson <[e-mail]>


r~

Patch

diff --git a/accel/tcg/cpu-exec.c b/accel/tcg/cpu-exec.c
index e661635f06..499a8bdc5e 100644
--- a/accel/tcg/cpu-exec.c
+++ b/accel/tcg/cpu-exec.c
@@ -556,7 +556,6 @@  static inline bool cpu_handle_interrupt(CPUState *cpu,
     if (unlikely(cpu_interrupt_request(cpu))) {
         int interrupt_request;
 
-        qemu_mutex_lock_iothread();
         interrupt_request = cpu_interrupt_request(cpu);
         if (unlikely(cpu->singlestep_enabled & SSTEP_NOIRQ)) {
             /* Mask out external interrupts for this step. */
@@ -565,7 +564,6 @@  static inline bool cpu_handle_interrupt(CPUState *cpu,
         if (interrupt_request & CPU_INTERRUPT_DEBUG) {
             cpu_reset_interrupt(cpu, CPU_INTERRUPT_DEBUG);
             cpu->exception_index = EXCP_DEBUG;
-            qemu_mutex_unlock_iothread();
             return true;
         }
         if (replay_mode == REPLAY_MODE_PLAY && !replay_has_interrupt()) {
@@ -575,13 +573,13 @@  static inline bool cpu_handle_interrupt(CPUState *cpu,
             cpu_reset_interrupt(cpu, CPU_INTERRUPT_HALT);
             cpu_halted_set(cpu, 1);
             cpu->exception_index = EXCP_HLT;
-            qemu_mutex_unlock_iothread();
             return true;
         }
 #if defined(TARGET_I386)
         else if (interrupt_request & CPU_INTERRUPT_INIT) {
             X86CPU *x86_cpu = X86_CPU(cpu);
             CPUArchState *env = &x86_cpu->env;
+            qemu_mutex_lock_iothread();
             replay_interrupt();
             cpu_svm_check_intercept_param(env, SVM_EXIT_INIT, 0, 0);
             do_cpu_init(x86_cpu);
@@ -593,7 +591,6 @@  static inline bool cpu_handle_interrupt(CPUState *cpu,
         else if (interrupt_request & CPU_INTERRUPT_RESET) {
             replay_interrupt();
             cpu_reset(cpu);
-            qemu_mutex_unlock_iothread();
             return true;
         }
 #endif
@@ -602,6 +599,7 @@  static inline bool cpu_handle_interrupt(CPUState *cpu,
            True when it is, and we should restart on a new TB,
            and via longjmp via cpu_loop_exit.  */
         else {
+            qemu_mutex_lock_iothread();
             if (cc->cpu_exec_interrupt(cpu, interrupt_request)) {
                 replay_interrupt();
                 /*
@@ -616,6 +614,7 @@  static inline bool cpu_handle_interrupt(CPUState *cpu,
             /* The target hook may have updated the 'cpu->interrupt_request';
              * reload the 'interrupt_request' value */
             interrupt_request = cpu_interrupt_request(cpu);
+            qemu_mutex_unlock_iothread();
         }
         if (interrupt_request & CPU_INTERRUPT_EXITTB) {
             cpu_reset_interrupt(cpu, CPU_INTERRUPT_EXITTB);
@@ -625,7 +624,6 @@  static inline bool cpu_handle_interrupt(CPUState *cpu,
         }
 
         /* If we exit via cpu_loop_exit/longjmp it is reset in cpu_exec */
-        qemu_mutex_unlock_iothread();
     }
 
     /* Finally, check if we need to exit to the main loop.  */