Re: [PATCH] riscv: Make semihosting configurable for all privilege modes

[Andrew Jones]

On Thu, Aug 11, 2022 at 01:41:04PM -0700, Furquan Shaikh wrote:
> Unlike ARM, RISC-V does not define a separate breakpoint type for
> semihosting. Instead, it is entirely ABI. Thus, we need an option
> to allow users to configure what the ebreak behavior should be for
> different privilege levels - M, S, U, VS, VU. As per the RISC-V
> privilege specification[1], ebreak traps into the execution
> environment. However, RISC-V debug specification[2] provides
> ebreak{m,s,u,vs,vu} configuration bits to allow ebreak behavior to
> be configured to trap into debug mode instead. This change adds
> settable properties for RISC-V CPUs - `ebreakm`, `ebreaks`, `ebreaku`,
> `ebreakvs` and `ebreakvu` to allow user to configure whether qemu
> should treat ebreak as semihosting traps or trap according to the
> privilege specification.
> 
> [1] 
> https://github.com/riscv/riscv-isa-manual/releases/download/draft-20220723-10eea63/riscv-privileged.pdf
> [2] 
> https://github.com/riscv/riscv-debug-spec/blob/release/riscv-debug-release.pdf
> 
> Signed-off-by: Furquan Shaikh <furquan@rivosinc.com>
> ---
>  target/riscv/cpu.c        |  8 ++++++++
>  target/riscv/cpu.h        |  7 +++++++
>  target/riscv/cpu_helper.c | 26 +++++++++++++++++++++++++-
>  3 files changed, 40 insertions(+), 1 deletion(-)
> 
> diff --git a/target/riscv/cpu.c b/target/riscv/cpu.c
> index ac6f82ebd0..082194652b 100644
> --- a/target/riscv/cpu.c
> +++ b/target/riscv/cpu.c
> @@ -953,6 +953,14 @@ static Property riscv_cpu_properties[] = {
>      DEFINE_PROP_BOOL("short-isa-string", RISCVCPU,
> cfg.short_isa_string, false),
> 
>      DEFINE_PROP_BOOL("rvv_ta_all_1s", RISCVCPU, cfg.rvv_ta_all_1s, false),
> +
> +    /* Debug spec */
> +    DEFINE_PROP_BOOL("ebreakm", RISCVCPU, cfg.ebreakm, true),
> +    DEFINE_PROP_BOOL("ebreaks", RISCVCPU, cfg.ebreaks, false),
> +    DEFINE_PROP_BOOL("ebreaku", RISCVCPU, cfg.ebreaku, false),
> +    DEFINE_PROP_BOOL("ebreakvs", RISCVCPU, cfg.ebreakvs, false),
> +    DEFINE_PROP_BOOL("ebreakvu", RISCVCPU, cfg.ebreakvu, false),
> +
>      DEFINE_PROP_END_OF_LIST(),
>  };
> 
> diff --git a/target/riscv/cpu.h b/target/riscv/cpu.h
> index 5c7acc055a..eee8e487a6 100644
> --- a/target/riscv/cpu.h
> +++ b/target/riscv/cpu.h
> @@ -454,6 +454,13 @@ struct RISCVCPUConfig {
>      bool epmp;
>      bool aia;
>      bool debug;
> +
> +    /* Debug spec */
> +    bool ebreakm;
> +    bool ebreaks;
> +    bool ebreaku;
> +    bool ebreakvs;
> +    bool ebreakvu;

There's only five of these, so having each separate probably makes the
most sense, but I wanted to point out that we could keep the properties
independent booleans, as we should, but still consolidate the values
into a single bitmap like we did for the sve vq bitmap for arm (see
cpu_arm_get/set_vq). Maybe worth considering?

>      uint64_t resetvec;
> 
>      bool short_isa_string;
> diff --git a/target/riscv/cpu_helper.c b/target/riscv/cpu_helper.c
> index 59b3680b1b..be09abbe27 100644
> --- a/target/riscv/cpu_helper.c
> +++ b/target/riscv/cpu_helper.c
> @@ -1314,6 +1314,30 @@ bool riscv_cpu_tlb_fill(CPUState *cs, vaddr
> address, int size,
> 
>      return true;
>  }
> +
> +static bool semihosting_enabled(RISCVCPU *cpu)
> +{
> +    CPURISCVState *env = &cpu->env;
> +
> +    switch (env->priv) {
> +    case PRV_M:
> +        return cpu->cfg.ebreakm;
> +    case PRV_S:
> +        if (riscv_cpu_virt_enabled(env)) {
> +            return cpu->cfg.ebreakvs;
> +        } else {
> +            return cpu->cfg.ebreaks;
> +        }
> +    case PRV_U:
> +        if (riscv_cpu_virt_enabled(env)) {
> +            return cpu->cfg.ebreakvu;
> +        } else {
> +            return cpu->cfg.ebreaku;
> +        }
> +    }
> +
> +    return false;
> +}
>  #endif /* !CONFIG_USER_ONLY */
> 
>  /*
> @@ -1342,7 +1366,7 @@ void riscv_cpu_do_interrupt(CPUState *cs)
>      target_ulong mtval2 = 0;
> 
>      if  (cause == RISCV_EXCP_SEMIHOST) {
> -        if (env->priv >= PRV_S) {
> +        if (semihosting_enabled(cpu)) {
>              do_common_semihosting(cs);
>              env->pc += 4;
>              return;
> --
> 2.34.1
>

Bitmap or no bitmap,

Reviewed-by: Andrew Jones <ajones@ventanamicro.com>