Re: [Qemu-devel] [PATCH v2 05/26] armv7m: add armv7m_excp_running_prio()

[Peter Maydell]

On 3 December 2015 at 00:18, Michael Davidsaver <address@hidden> wrote:
> Implements v7m exception priority algorithm
> using FAULTMASK, PRIMASK, BASEPRI, and the highest
> priority active exception.
>
> The number returned is the current execution priority
> which may be in the range [-2,0x7f] when an exception is active
> or 0x100 when no exception is active.
> ---
>  hw/intc/armv7m_nvic.c | 25 +++++++++++++++++++++++++
>  target-arm/cpu.h      |  1 +
>  2 files changed, 26 insertions(+)
>
> diff --git a/hw/intc/armv7m_nvic.c b/hw/intc/armv7m_nvic.c
> index 6fc167e..0145ca7 100644
> --- a/hw/intc/armv7m_nvic.c
> +++ b/hw/intc/armv7m_nvic.c
> @@ -18,6 +18,8 @@
>
>  typedef struct {
>      GICState gic;
> +    uint8_t prigroup;
> +
>      struct {
>          uint32_t control;
>          uint32_t reload;
> @@ -116,6 +118,29 @@ static void systick_reset(nvic_state *s)
>      timer_del(s->systick.timer);
>  }
>
> +/* @returns the active (running) exception priority.
> + *    only a higher (numerically lower) priority can preempt.
> + */
> +int armv7m_excp_running_prio(ARMCPU *cpu)
> +{
> +    CPUARMState *env = &cpu->env;
> +    nvic_state *s = env->nvic;
> +    int running;
> +
> +    if (env->daif & PSTATE_F) { /* FAULTMASK */
> +        running = -1;
> +    } else if (env->daif & PSTATE_I) { /* PRIMASK */
> +        running = 0;
> +    } else if (env->v7m.basepri > 0) {
> +        /* BASEPRI==1 -> masks [1,255] (not same as PRIMASK==1) */
> +        running = env->v7m.basepri >> (s->prigroup+1);

This isn't right -- the effect of PRIGROUP is that we mask
out the lower (subgroup) bits, but the upper group bits stay
where they are rather than shifting down.

So you want env->v7m.basepri & ~((1 << (s->prigroup + 1)) - 1);

(the same mask you need to get the group priority for
an interrupt).

> +    } else {
> +        running = 0x100; /* lower than any possible priority */
> +    }
> +    /* consider priority of active handler */
> +    return MIN(running, env->v7m.exception_prio);
> +}
> +

thanks
-- PMM