Re: [Qemu-devel] TCG icount interaction with timer deadlines

[Peter Maydell]

On 5 April 2018 at 18:07, Paolo Bonzini <address@hidden> wrote:
> On 05/04/2018 18:01, Peter Maydell wrote:
>>  * however, if the guest reprograms the clock during the tcg_cpu_exec()
>>    run, we don't do anything to cause us to stop earlier
>
> Anything that does this from the vCPU thread should be between
> gen_icount_start and gen_icount_end.  (In fact, it's the entire reason
> why cpu_io_recompile exists).

Yes, and this does cause us to do a cpu_io_recompile, which
rebuilds the TB and does a longjmp. However:
 (1) that only takes us out to cpu_exec(), which will then
 just go ahead and execute the next TB, whereas the
 recalculation of deadlines happens at the next level out
 in tcg_cpu_exec()
 (2) the io_recompile happens *before* the guest writes to
 the timer register that reprograms the deadline, so even
 if we recomputed deadlines after this longjmp they wouldn't
 be correct

thanks
-- PMM