Re: [Qemu-devel] [PATCH 3/3] arm: semihosting: Wire up A64 HLT 0xf000

[Peter Maydell]

On 27 March 2015 at 16:22, Christopher Covington
<address@hidden> wrote:
> In AArch64, Angel semihosting uses HLT instructions with a special
> immediate value, 0xf000. Use a new exception type EXCP_SEMI for this,
> since I'm not aware of plans for full HLT support, and EXCP_HLT is
> already defined as a QEMU internal exception type. Support just the
> exit call in A64 to start with.
>
> Signed-off-by: Christopher Covington <address@hidden>
> ---
>  target-arm/arm-semi.c      | 14 +++++++++++---
>  target-arm/cpu.h           |  3 ++-
>  target-arm/helper-a64.c    | 11 +++++++++++
>  target-arm/internals.h     |  1 +
>  target-arm/translate-a64.c |  6 +++++-
>  5 files changed, 30 insertions(+), 5 deletions(-)
>
> diff --git a/target-arm/arm-semi.c b/target-arm/arm-semi.c
> index d915123..c71d47a 100644
> --- a/target-arm/arm-semi.c
> +++ b/target-arm/arm-semi.c
> @@ -191,7 +191,7 @@ static void QEMU_NORETURN 
> unsupported_semihosting(uint32_t nr, CPUState *cs)
>  } while (0)
>
>  #define SET_ARG(n, val) put_user_ual(val, args + (n) * 4)
> -uint32_t do_arm_semihosting(CPUARMState *env)
> +target_ulong do_arm_semihosting(CPUARMState *env)
>  {
>      ARMCPU *cpu = arm_env_get_cpu(env);
>      CPUState *cs = CPU(cpu);
> @@ -207,8 +207,16 @@ uint32_t do_arm_semihosting(CPUARMState *env)
>      CPUARMState *ts = env;
>  #endif
>
> -    nr = env->regs[0];
> -    args = env->regs[1];
> +    if (is_a64(env)) {
> +        nr = env->xregs[0];

> +        args = env->xregs[1];
> +        if (nr != env->xregs[0] || nr != TARGET_SYS_EXIT) {

What is the first part of this if condition intended to do?
(Note that the semihosting API number is passed in W0,
not X0...)

> +            unsupported_semihosting(nr, cs);
> +        }
> +    } else {
> +        nr = env->regs[0];
> +        args = env->regs[1];
> +    }
>      switch (nr) {
>      case TARGET_SYS_OPEN:
>          GET_ARG(0);

This doesn't look right -- in the AArch64 version
the SYS_EXIT (aka AngelSWI_Reason_ReportException)
takes a parameter block, so you can't just fall through
to the existing code without modifying it to support this.

It's also a bit difficult to tell if this code is
right without seeing some actual implementations of
the semihosting calls.

> diff --git a/target-arm/cpu.h b/target-arm/cpu.h
> index 083211c..d5c5cfa 100644
> --- a/target-arm/cpu.h
> +++ b/target-arm/cpu.h
> @@ -56,6 +56,7 @@
>  #define EXCP_SMC            13   /* Secure Monitor Call */
>  #define EXCP_VIRQ           14
>  #define EXCP_VFIQ           15
> +#define EXCP_SEMI           16   /* Angel Semihosting Call */
>
>  #define ARMV7M_EXCP_RESET   1
>  #define ARMV7M_EXCP_NMI     2
> @@ -494,7 +495,7 @@ typedef struct CPUARMState {
>
>  ARMCPU *cpu_arm_init(const char *cpu_model);
>  int cpu_arm_exec(CPUARMState *s);
> -uint32_t do_arm_semihosting(CPUARMState *env);
> +target_ulong do_arm_semihosting(CPUARMState *env);
>  void aarch64_sync_32_to_64(CPUARMState *env);
>  void aarch64_sync_64_to_32(CPUARMState *env);
>
> diff --git a/target-arm/helper-a64.c b/target-arm/helper-a64.c
> index 7e0d038..9ecd488 100644
> --- a/target-arm/helper-a64.c
> +++ b/target-arm/helper-a64.c
> @@ -514,6 +514,17 @@ void aarch64_cpu_do_interrupt(CPUState *cs)
>      case EXCP_VFIQ:
>          addr += 0x100;
>          break;
> +    case EXCP_SEMI:
> +        if (semihosting_enabled) {
> +            qemu_log_mask(CPU_LOG_INT,
> +                          "...handling as semihosting call 0x%" PRIx64 "\n",
> +                          env->xregs[0]);
> +            env->xregs[0] = do_arm_semihosting(env);
> +            break;
> +        } else {
> +            qemu_log_mask(CPU_LOG_INT,
> +                          "...not handled because semihosting is 
> disabled\n");
> +        }
>      default:
>          cpu_abort(cs, "Unhandled exception 0x%x\n", cs->exception_index);
>      }
> diff --git a/target-arm/internals.h b/target-arm/internals.h
> index bb171a7..86b5854 100644
> --- a/target-arm/internals.h
> +++ b/target-arm/internals.h
> @@ -58,6 +58,7 @@ static const char * const excnames[] = {
>      [EXCP_SMC] = "Secure Monitor Call",
>      [EXCP_VIRQ] = "Virtual IRQ",
>      [EXCP_VFIQ] = "Virtual FIQ",
> +    [EXCP_SEMI] = "Angel Semihosting Call",
>  };
>
>  static inline void arm_log_exception(int idx)
> diff --git a/target-arm/translate-a64.c b/target-arm/translate-a64.c
> index 0b192a1..3b5b875 100644
> --- a/target-arm/translate-a64.c
> +++ b/target-arm/translate-a64.c
> @@ -1544,7 +1544,11 @@ static void disas_exc(DisasContext *s, uint32_t insn)
>              break;
>          }
>          /* HLT */
> -        unsupported_encoding(s, insn);
> +        if (imm16 == 0xf000) {

You need to have the semihosting_enabled check here rather
than in the do_interrupt code, because otherwise we won't
behave correctly in the disabled case.

> +            gen_exception_insn(s, 4, EXCP_SEMI, 0);

> +        } else {
> +            unsupported_encoding(s, insn);
> +        }
>          break;
>      case 5:
>          if (op2_ll < 1 || op2_ll > 3) {
> --
> 1.9.1

thanks
-- PMM