[PATCH v1 04/21] target/avr: add BQL to do_interrupt and cpu_exec_interrupt

[Robert Foley]

This is part of a series of changes to remove the implied BQL
from the common code of cpu_handle_interrupt and
cpu_handle_exception.  As part of removing the implied BQL
from the common code, we are pushing the BQL holding
down into the per-arch implementation functions of
do_interrupt and cpu_exec_interrupt.

The purpose of this set of changes is to set the groundwork
so that an arch could move towards removing
the BQL from the cpu_handle_interrupt/exception paths.

This approach was suggested by Paolo Bonzini.
For reference, here are two key posts in the discussion, explaining
the reasoning/benefits of this approach.
https://lists.gnu.org/archive/html/qemu-devel/2020-07/msg08731.html
https://lists.gnu.org/archive/html/qemu-devel/2020-08/msg00044.html

Signed-off-by: Robert Foley <robert.foley@linaro.org>
---
 target/avr/helper.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/target/avr/helper.c b/target/avr/helper.c
index d96d14372b..f0d625c195 100644
--- a/target/avr/helper.c
+++ b/target/avr/helper.c
@@ -30,6 +30,7 @@ bool avr_cpu_exec_interrupt(CPUState *cs, int 
interrupt_request)
     CPUClass *cc = CPU_GET_CLASS(cs);
     AVRCPU *cpu = AVR_CPU(cs);
     CPUAVRState *env = &cpu->env;
+    qemu_mutex_lock_iothread();
 
     if (interrupt_request & CPU_INTERRUPT_RESET) {
         if (cpu_interrupts_enabled(env)) {
@@ -53,6 +54,7 @@ bool avr_cpu_exec_interrupt(CPUState *cs, int 
interrupt_request)
             ret = true;
         }
     }
+    qemu_mutex_unlock_iothread();
     return ret;
 }
 
@@ -61,10 +63,15 @@ void avr_cpu_do_interrupt(CPUState *cs)
     AVRCPU *cpu = AVR_CPU(cs);
     CPUAVRState *env = &cpu->env;
 
-    uint32_t ret = env->pc_w;
+    uint32_t ret;
     int vector = 0;
     int size = avr_feature(env, AVR_FEATURE_JMP_CALL) ? 2 : 1;
     int base = 0;
+    bool bql = !qemu_mutex_iothread_locked();
+    if (bql) {
+        qemu_mutex_lock_iothread();
+    }
+    ret = env->pc_w;
 
     if (cs->exception_index == EXCP_RESET) {
         vector = 0;
@@ -87,6 +94,9 @@ void avr_cpu_do_interrupt(CPUState *cs)
     env->sregI = 0; /* clear Global Interrupt Flag */
 
     cs->exception_index = -1;
+    if (bql) {
+        qemu_mutex_unlock_iothread();
+    }
 }
 
 int avr_cpu_memory_rw_debug(CPUState *cs, vaddr addr, uint8_t *buf,
-- 
2.17.1