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Re: [Qemu-devel] [PULL 07/11] cpu-exec: don't overwrite exception_index

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PULL 07/11] cpu-exec: don't overwrite exception_index
Date: Mon, 20 Nov 2017 22:08:14 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

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->exception_index = -1;
        return true;
    } else {
        CPUClass *cc = CPU_GET_CLASS(cpu);
        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.



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