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/mips/helper.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/target/mips/helper.c b/target/mips/helper.c
index afd78b1990..6595d18702 100644
--- a/target/mips/helper.c
+++ b/target/mips/helper.c
@@ -1085,6 +1085,10 @@ static inline void set_badinstr_registers(CPUMIPSState *env)
 
 void mips_cpu_do_interrupt(CPUState *cs)
 {
+    bool bql = !qemu_mutex_iothread_locked();
+    if (bql) {
+        qemu_mutex_lock_iothread();
+    }
 #if !defined(CONFIG_USER_ONLY)
     MIPSCPU *cpu = MIPS_CPU(cs);
     CPUMIPSState *env = &cpu->env;
@@ -1396,10 +1400,14 @@ void mips_cpu_do_interrupt(CPUState *cs)
     }
 #endif
     cs->exception_index = EXCP_NONE;
+    if (bql) {
+        qemu_mutex_unlock_iothread();
+    }
 }
 
 bool mips_cpu_exec_interrupt(CPUState *cs, int interrupt_request)
 {
+    qemu_mutex_lock_iothread();
     if (interrupt_request & CPU_INTERRUPT_HARD) {
         MIPSCPU *cpu = MIPS_CPU(cs);
         CPUMIPSState *env = &cpu->env;
@@ -1410,9 +1418,11 @@ bool mips_cpu_exec_interrupt(CPUState *cs, int interrupt_request)
             cs->exception_index = EXCP_EXT_INTERRUPT;
             env->error_code = 0;
             mips_cpu_do_interrupt(cs);
+            qemu_mutex_unlock_iothread();
             return true;
         }
     }
+    qemu_mutex_unlock_iothread();
     return false;
 }
 
-- 
2.17.1