Re: [RFC PATCH] docs/devel: add some notes on tcg-icount for developers

[Peter Maydell]

On Fri, 19 Jun 2020 at 14:58, Alex Bennée <alex.bennee@linaro.org> wrote:
>
> This attempts to bring together my understanding of the requirements
> for icount behaviour into one reference document for our developer
> notes. It currently make one piece of conjecture which I think is true
> that we don't need gen_io_start/end statements for non-MMIO related
> I/O operations.

> +Other I/O operations
> +--------------------
> +
> +MMIO isn't the only type of operation for which we might need a
> +correct and accurate clock. IO port instructions and accesses to
> +system registers are the common examples here. For the clock to be
> +accurate you end a translation block on these instructions.
> +
> +.. warning:: (CONJECTURE) instructions that won't get trapped in the
> +             io_read/writex shouldn't need gen_io_start/end blocks
> +             around them.

I think this is backwards -- instructions where icount is handled
by io_readx/io_writex are the ones that don't need to be marked
with gen_io_start. It's the i/o instructions that don't go through
io_readx/io_writex that need gen_io_start.

thanks
-- PMM