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From: "Pavel Dovgalyuk" <dovgaluk@ispras.ru>
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Date: Thu, 26 Jan 2017 17:32:37 +0300
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Subject: Re: [Qemu-devel] [PATCH v8 2/9] icount: exit cpu loop on expire
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To: 'Paolo Bonzini' <pbonzini@redhat.com>, 'Pavel Dovgalyuk' <pavel.dovgaluk@ispras.ru>, qemu-devel@nongnu.org
Cc: kwolf@redhat.com, peter.maydell@linaro.org, quintela@redhat.com, jasowang@redhat.com, mst@redhat.com, kraxel@redhat.com

> From: Paolo Bonzini [mailto:pbonzini@redhat.com]
> On 26/01/2017 14:37, Pavel Dovgalyuk wrote:
> >> Simpler:
> >>
> >> 	use_icount &&
> >> 	((int32_t)cpu->icount_decr.u32 < 0 ||
> >> 	 cpu->icount_decr.u16.low + cpu->icount_extra == 0)
> > Right.
> >
> >> But I'm not sure that you need to test u32.  After all you're not
> > Checking u32 is needed, because sometimes it is less than zero.
> 
> If cpu->icount_decr.u32 is less than zero, the next translation block
> would immediately exit with TB_EXIT_ICOUNT_EXPIRED, causing
> 
>             cpu->exception_index = EXCP_INTERRUPT;
>             *last_tb = NULL;
>             cpu_loop_exit(cpu);
> 
> from cpu_loop_exec_tb's "case TB_EXIT_ICOUNT_EXPIRED".
> 
> And the same is true for cpu->icount_decr.u16.low + cpu->icount_extra ==
> 0, so I don't understand why this part of the patch is necessary.

I removed that lines because we have to check icount=0 not only when it is expired,
but also when all instructions were executed successfully. 
If there are no instructions to execute, calling tb_find (and translation then)
may cause an exception at the wrong moment.

Pavel Dovgalyuk