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/nios2/cpu.c    | 3 +++
 target/nios2/helper.c | 8 +++++++-
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/target/nios2/cpu.c b/target/nios2/cpu.c
index fe5fd9adfd..fd05406eac 100644
--- a/target/nios2/cpu.c
+++ b/target/nios2/cpu.c
@@ -102,13 +102,16 @@ static bool nios2_cpu_exec_interrupt(CPUState *cs, int interrupt_request)
 {
     Nios2CPU *cpu = NIOS2_CPU(cs);
     CPUNios2State *env = &cpu->env;
+    qemu_mutex_lock_iothread();
 
     if ((interrupt_request & CPU_INTERRUPT_HARD) &&
         (env->regs[CR_STATUS] & CR_STATUS_PIE)) {
         cs->exception_index = EXCP_IRQ;
         nios2_cpu_do_interrupt(cs);
+        qemu_mutex_unlock_iothread();
         return true;
     }
+    qemu_mutex_unlock_iothread();
     return false;
 }
 
diff --git a/target/nios2/helper.c b/target/nios2/helper.c
index 57c97bde3c..46d53551d4 100644
--- a/target/nios2/helper.c
+++ b/target/nios2/helper.c
@@ -52,7 +52,10 @@ void nios2_cpu_do_interrupt(CPUState *cs)
 {
     Nios2CPU *cpu = NIOS2_CPU(cs);
     CPUNios2State *env = &cpu->env;
-
+    bool bql = !qemu_mutex_iothread_locked();
+    if (bql) {
+        qemu_mutex_lock_iothread();
+    }
     switch (cs->exception_index) {
     case EXCP_IRQ:
         assert(env->regs[CR_STATUS] & CR_STATUS_PIE);
@@ -198,6 +201,9 @@ void nios2_cpu_do_interrupt(CPUState *cs)
                   cs->exception_index);
         break;
     }
+    if (bql) {
+        qemu_mutex_unlock_iothread();
+    }
 }
 
 hwaddr nios2_cpu_get_phys_page_debug(CPUState *cs, vaddr addr)
-- 
2.17.1