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

[Pavel Dovgalyuk]

> From: Paolo Bonzini [mailto:address@hidden
> 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.
>          */
>         smb_mb();
>     }
> 
> 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.
>          */
>         return;
>     }

I tried this approach and it didn't work.
I think iothread sets u16.high flag after resetting it in cpu_handle_interrupt.

But there is another possible approach: set new tcg flag, which disables 
checking
the exit_request at the entry to the TB.


Pavel Dovgalyuk