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/riscv/cpu_helper.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/target/riscv/cpu_helper.c b/target/riscv/cpu_helper.c
index 75d2ae3434..5050802e95 100644
--- a/target/riscv/cpu_helper.c
+++ b/target/riscv/cpu_helper.c
@@ -80,14 +80,17 @@ bool riscv_cpu_exec_interrupt(CPUState *cs, int interrupt_request)
 {
 #if !defined(CONFIG_USER_ONLY)
     if (interrupt_request & CPU_INTERRUPT_HARD) {
+        qemu_mutex_lock_iothread();
         RISCVCPU *cpu = RISCV_CPU(cs);
         CPURISCVState *env = &cpu->env;
         int interruptno = riscv_cpu_local_irq_pending(env);
         if (interruptno >= 0) {
             cs->exception_index = RISCV_EXCP_INT_FLAG | interruptno;
             riscv_cpu_do_interrupt(cs);
+            qemu_mutex_unlock_iothread();
             return true;
         }
+        qemu_mutex_unlock_iothread();
     }
 #endif
     return false;
@@ -822,6 +825,10 @@ bool riscv_cpu_tlb_fill(CPUState *cs, vaddr address, int size,
  */
 void riscv_cpu_do_interrupt(CPUState *cs)
 {
+    bool bql = !qemu_mutex_iothread_locked();
+    if (bql) {
+        qemu_mutex_lock_iothread();
+    }
 #if !defined(CONFIG_USER_ONLY)
 
     RISCVCPU *cpu = RISCV_CPU(cs);
@@ -982,4 +989,7 @@ void riscv_cpu_do_interrupt(CPUState *cs)
 
 #endif
     cs->exception_index = EXCP_NONE; /* mark handled to qemu */
+    if (bql) {
+        qemu_mutex_unlock_iothread();
+    }
 }
-- 
2.17.1