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From: Peter Maydell <peter.maydell@linaro.org>
Date: Fri, 27 Jun 2014 11:54:47 +0100
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Subject: Re: [Qemu-devel] Reverse execution and deterministic replay
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To: Pavel Dovgaluk <Pavel.Dovgaluk@ispras.ru>
Cc: Paolo Bonzini <pbonzini@redhat.com>, Peter Crosthwaite <peter.crosthwaite@xilinx.com>, Mark Burton <mark.burton@greensocs.com>, QEMU Developers <qemu-devel@nongnu.org>, Frederic Konrad <fred.konrad@greensocs.com>

On 27 June 2014 11:35, Pavel Dovgaluk <Pavel.Dovgaluk@ispras.ru> wrote:
> The major disadvantage of icount is that it's updated only on TB boundaries.
> When one instruction in the middle of the block uses virtual clock, it could
> have different values for different divisions of the code to TB.

This is only true if the instruction is incorrectly not
marked as being "I/O". The idea behind icount is that in
general we update it on TB boundaries (it's much faster
than doing it once per insn) but for those places which
do turn out to need an exact icount we then retranslate
the block to get the instruction-to-icount-adjustment
mapping.

It wouldn't surprise me if this turned out to have some
bugs in corner cases, but fixing these issues seems to
me like a much better design than ignoring icount completely
and reimplementing a second instruction counter.

thanks
-- PMM