Re: [Qemu-devel] when does a target frontend need to use gen_io_start()/gen_io_end() ?

[Peter Maydell]

On 12 May 2015 at 16:43, Richard Henderson <address@hidden> wrote:
> On 05/12/2015 08:32 AM, Peter Maydell wrote:
>> In order for -icount to work, it's important for the target
>> translate.c code to correctly bracket any generated code which
>> can "do I/O" with gen_io_start()/gen_io_end() calls. But
>> does anybody know exactly what the criteria are here for this?
>> It would be nice if we could document this in a comment in
>> gen_icount.h -- I'm happy to write one up if somebody will just
>> tell me what the right answer is :-)
>
> I'm really not sure.
>
> So far I've assumed "i/o"-like insns, and those that can read some sort of
> cycle counter.  So while that handles easy cases like "inb" and "rdcc", it
> certainly doesn't handle any target for which all i/o is memory mapped.

I think the "mmio access" case is already dealt with in the
softmmu_template.h handlers, isn't it? If the CPU isn't in a
"can do IO" state then the io_read/write handlers call
cpu_io_recompile(), which figures out how far through the TB
we were (using the machinery we already have for converting
host addresses of faults into guest PC values), and creates
a new TB which stops with the MMIO load/store. (I don't
entirely understand cpu_io_recompile(), though -- it looks
rather tricksy.)

-- PMM