Microblaze libgloss and gdb simulator

[Mike Frysinger]

On 22 Apr 2021 19:55, Joel Sherrill wrote:
> On Thu, Apr 22, 2021, 7:27 PM Mike Frysinger wrote:
> > ignoring that, the microblaze sim doesn't have syscall support hooked up.
> > so it's only a CPU simulator atm.
> 
> So it has no output whatsoever? Does it get used for anything?

afaict, correct.  the most basic sims just do CPU level stuff and then have
their state inspected, or communicate pass/fail via exit status or abort.
this behavior actually isn't that rare ... it's where most sims start.

> We are resurrecting some old work that I did for a Microblaze port. I did
> write an inbyte() and outbyte() which would normally come from the xil
> library. But I don't have any idea how I figured out there was a uart at a
> particular address. I swear I had it working to print then but now it
> faults after the first instruction.
> 
> Is there any known good executable for it? Even just seeing it operate with
> a linked executable that had a crt0 and did something would be helpful at
> this point.

ftr, i've never worked on microblaze.  i'm just reading the code and poking
the toolchain :).

getting i/o (or maybe just o) support into the sim shouldn't be terribly hard.
we could even do the normal libgloss syscalls.  the important things we need to
know are:
* how does outbyte work ?  is it writing to MMIO UARTs, or something else ?
* is there an interrupt or exception or specific insn that microblaze uses to
  trigger the hypervisor/monitor/whatever ?  if so, should be possible to wire
  that up in the microblaze port.  my reading of libgloss/microblaze/ isn't
  picking out anything interesting, but i'm by no means an expert here.

if you can figure out those bits, happy to help on the sim side.
-mike