RE: [PATCH 3/3] target/nios2: Use gen_io_start around wrctl instruction

[Wu, Wentong]

Maybe below patch will reduce some overhead, because currently it will exit to 
main loop to handle interrupt but if with (env->regs[CR_STATUS] & 
CR_STATUS_PIE) = False, it does nothing except set env->irq_pending again.

diff --git a/hw/nios2/cpu_pic.c b/hw/nios2/cpu_pic.c
index 1c1989d5..5ea7e52a 100644
--- a/hw/nios2/cpu_pic.c
+++ b/hw/nios2/cpu_pic.c
@@ -54,7 +54,8 @@ static void nios2_pic_cpu_handler(void *opaque, int irq, int 
level)

 void nios2_check_interrupts(CPUNios2State *env)
 {
-    if (env->irq_pending) {
+    if (env->irq_pending &&
+        (env->regs[CR_STATUS] & CR_STATUS_PIE)) {
         env->irq_pending = 0;
         cpu_interrupt(env_cpu(env), CPU_INTERRUPT_HARD);
     }

-----Original Message-----
From: Richard Henderson <richard.henderson@linaro.org> 
Sent: Friday, July 3, 2020 2:54 AM
To: Wu, Wentong <wentong.wu@intel.com>; qemu-devel@nongnu.org
Cc: qemu-trivial@nongnu.org; marex@denx.de; crwulff@gmail.com; 
peter.maydell@linaro.org
Subject: Re: [PATCH 3/3] target/nios2: Use gen_io_start around wrctl instruction

On 6/29/20 9:05 AM, Wentong Wu wrote:
> wrctl instruction on nios2 target will cause checking cpu interrupt 
> but tcg_handle_interrupt() will call cpu_abort() if the CPU gets an 
> interrupt while it's not in 'can do IO'
> state, so add gen_io_start around wrctl instruction. Also at the same 
> time, end the onging TB with DISAS_UPDATE.
> 
> Signed-off-by: Wentong Wu <wentong.wu@intel.com>
> ---
>  target/nios2/translate.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/target/nios2/translate.c b/target/nios2/translate.c index 
> 83c10eb2..51347ada 100644
> --- a/target/nios2/translate.c
> +++ b/target/nios2/translate.c
> @@ -32,6 +32,7 @@
>  #include "exec/cpu_ldst.h"
>  #include "exec/translator.h"
>  #include "qemu/qemu-print.h"
> +#include "exec/gen-icount.h"
>  
>  /* is_jmp field values */
>  #define DISAS_JUMP    DISAS_TARGET_0 /* only pc was modified dynamically */
> @@ -518,7 +519,11 @@ static void wrctl(DisasContext *dc, uint32_t code, 
> uint32_t flags)
>      /* If interrupts were enabled using WRCTL, trigger them. */  #if 
> !defined(CONFIG_USER_ONLY)
>      if ((instr.imm5 + CR_BASE) == CR_STATUS) {
> +        if (tb_cflags(dc->tb) & CF_USE_ICOUNT) {
> +            gen_io_start();
> +        }
>          gen_helper_check_interrupts(dc->cpu_env);
> +        dc->is_jmp = DISAS_UPDATE;
>      }
>  #endif

This isn't right.  Not so much the gen_io_start portion, but the entire 
existence of helper_check_interrupt.

The correct way to acknowledge interrupts after changing an interrupt mask bit 
is to exit the TB back to the cpu main loop.
Which you are doing here with DISAS_UPDATE, so that part is fine.  (Although 
you could merge that into the switch statement above.)

Looking at nios_pic_cpu_handler, there are two other bugs:

1) Get rid of env->irq_pending and use cpu_interrupt/cpu_reset_interrupt 
instead.

2) Do not check env->regs[CR_STATUS] & CR_STATUS_PIE.  That variable does not 
belong to the pic and should not be checked there.  The check belongs in 
nios2_cpu_exec_interrupt, and is in fact already there.


r~