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From: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <619db202-f919-c8e9-c881-ff08373f287d@redhat.com>
Date: Mon, 20 Nov 2017 22:08:14 +0100
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Subject: Re: [Qemu-devel] [PULL 07/11] cpu-exec: don't overwrite
 exception_index
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To: Peter Maydell <peter.maydell@linaro.org>, Pavel Dovgalyuk <dovgaluk@ispras.ru>
Cc: QEMU Developers <qemu-devel@nongnu.org>, Pavel Dovgalyuk <pavel.dovgaluk@ispras.ru>

On 20/11/2017 13:50, Peter Maydell wrote:
> More generally, this commit seems to assume that QEMU always
> does:
>  * set exception_index to something
>  * handle that
>  * clear exception_index to -1
> 
> but it's not clear to me that it's actually always the case
> that it gets cleared back to -1.

After returning from cpu_handle_interrupt, cpu_exec goes to
cpu_handle_exception which does

    if (cpu->exception_index >= EXCP_INTERRUPT) {
        *ret = cpu->exception_index;
        if (*ret == EXCP_DEBUG) {
            cpu_handle_debug_exception(cpu);
        }
        cpu->exception_index = -1;
        return true;
    } else {
        CPUClass *cc = CPU_GET_CLASS(cpu);
        qemu_mutex_lock_iothread();
        cc->do_interrupt(cpu);
        qemu_mutex_unlock_iothread();
        cpu->exception_index = -1;
    }

    return false;

Does ARM have a case where cc->do_interrupt can longjmp back to the
beginning of cpu_handle_exception?  But I still do not understand why
you don't eventually clear exception_index to -1.  Maybe there should be
an assertion for that before and after cpu_handle_interrupt.

Thanks,

Paolo