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Re: [Qemu-devel] [RFC PATCH 19/26] cpu-exec: reset exit flag before call

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [RFC PATCH 19/26] cpu-exec: reset exit flag before calling cpu_exec_nocache
Date: Thu, 2 Nov 2017 12:46:35 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

On 02/11/2017 12:33, Paolo Bonzini wrote:
> On 02/11/2017 12:24, Pavel Dovgalyuk wrote:
>>> I am not sure about this.  I think if instead you should return false
>>> from here and EXCP_INTERRUPT from cpu_exec.
>> The problem is inside the TB. It checks cpu->icount_decr.u16.high which is 
>> -1.
>> And we have to enter the TB to cause an exception (because it exists in 
>> replay log).
>> That is why we reset this flag and try to execute the TB.
> But if u16.high is -1, shouldn't you return EXCP_INTERRUPT first (via
> "Finally, check if we need to exit to the main loop" in
> cpu_handle_interrupt)?  Then only cause the exception when that one is
> processed.

... indeed, you probably need something like:

    /* Clear the interrupt flag now since we're processing
     * cpu->interrupt_request and cpu->exit_request.
    insns_left = atomic_read(&cpu->icount_decr.u32);
    atomic_set(&cpu->icount_decr.u16.high, 0);
    if (unlikely(insns_left < 0) {
        /* Ensure the zeroing of icount_decr comes before the next read
         * of cpu->exit_request or cpu->interrupt_request.

at the top of cpu_handle_interrupt.  Then you can remove the same
atomic_set+smp_mb in cpu_loop_exec_tb, like

    *last_tb = NULL;
    insns_left = atomic_read(&cpu->icount_decr.u32);
    if (insns_left < 0) {
        /* Something asked us to stop executing chained TBs; just
         * continue round the main loop. Whatever requested the exit
         * will also have set something else (eg exit_request or
         * interrupt_request) which will be handled by
         * cpu_handle_interrupt.  cpu_handle_interrupt will also
         * clear cpu->icount_decr.u16.high.



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