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From: Richard Henderson <rth@twiddle.net>
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Date: Tue, 16 Feb 2016 07:52:14 +1100
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Subject: Re: [Qemu-devel] [PATCH v2 1/4] target-tricore: Add trap handling
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To: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>, qemu-devel@nongnu.org

On 02/16/2016 12:10 AM, Bastian Koppelmann wrote:
> +
> +void tricore_cpu_do_interrupt(CPUState *cs)
> +{
> +    TriCoreCPU *cpu = TRICORE_CPU(cs);
> +    CPUTriCoreState *env = &cpu->env;
> +
> +    if (cs->exception_index <= TRAPC_NMI) {
> +        /* The trap vector table is accessed to fetch the first instruction of
> +           the trap handler. */
> +        env->PC = env->BTV | (cs->exception_index << 5);
> +    } else if (cs->exception_index == TRAPC_IRQ) {
> +        /* The interrupt vector table is accessed to fetch the first instruction
> +           of the interrupt handler. */
> +        env->PC = env->BIV | ((env->ICR & MASK_ICR_PIPN) >> 10);
> +    }
> +}

You've still got a path whereby you modify PC without saving the old one.

I don't think you want to add the do_interrupt hook at all until you're ready 
to do real async interrupts.

> +    /* PCXI.PCPN = ICR.CCPN */
> +    env->PCXI = (env->PCXI & 0xffffff) +
> +                ((env->ICR & MASK_ICR_CCPN) << 24);
> +    cs->exception_index = class;
> +    cpu_loop_exit(cs);
> +}
> +

There's no reason you can't modify PC here at the end of 
raise_exception_sync_internal.  If you omit the set of exception_index here, 
you'll simply exit the cpu loop and immediately re-enter it at the new PC, 
without having to go through the do_interrupt hook.


r~