Re: [PATCH v5 2/2] target/xtensa: Use semihosting/syscalls.h

[Alex Bennée]

Max Filippov <jcmvbkbc@gmail.com> writes:

> On Wed, Jun 29, 2022 at 1:09 AM Alex Bennée <alex.bennee@linaro.org> wrote:
>> Richard Henderson <richard.henderson@linaro.org> writes:
>> > On 6/28/22 19:08, Max Filippov wrote:
>> >> On Tue, Jun 28, 2022 at 4:43 AM Richard Henderson
>> >> <richard.henderson@linaro.org> wrote:
>
>> >>>       }
>> >>> -    if (serial_hd(0)) {
>> >>> -        xtensa_sim_open_console(serial_hd(0));
>> >>> -    }
>> >> I've noticed that with this change '-serial stdio' and its variants
>> >> are still
>> >> accepted in the command line, but now they do nothing.
>> >
>> > Pardon?  They certainly will do something, via writes to the serial 
>> > hardware.
>> >
>> >
>> >> This quiet
>> >> change of behavior is unfortunate. I wonder if it would be acceptable
>> >> to map the '-serial stdio' option in the presence of '-semihosting' to
>> >> something like '-chardev stdio,id=id1 -semihosting-config chardev=id1'?
>> >
>> > I dunno.  I'm wary of having xtensa be unique here.  Alex, thoughts?
>>
>> Is semihosting *the* serial hardware for xtensa-sim or is it overriding
>> another serial interface? I'm wary of adding more magical behaviour for
>> -serial as it can be confusing enough already what actually gets routed
>> to it if not doing everything explicitly.
>
> There's no notion of 'serial hardware' for the xtensa-sim, all it has is
> the three standard stdio file descriptors.

Which are accessed via semihosting calls? Are they implicitly mapped to
3 chardev devices for stdin, stdout and stderr?

> But it was convenient thinking
> of them as a serial port. I agree that no magic is needed here, but
> the change shouldn't be quiet eiter, so xtensa-sim should warn (or
> maybe even quit with an error code) when it sees the -serial option.

If the default chardevs already map to the 3 FDs then perhaps -serial
should be invalid because it is more explicit to use -chardev to
redirect the stream you want somewhere else. However I don't see them at
the moment:

  ➜  ./qemu-system-xtensa -M sim -semihosting -S -display none -monitor stdio
  QEMU 7.0.50 monitor - type 'help' for more information
  (qemu) info chardev 
  compat_monitor0: filename=stdio
  parallel0: filename=vc

-- 
Alex Bennée