[PATCH] sim: add more hacking notes

[Mike Frysinger <vapier at gentoo dot org>]

I found the documentation lacking in many places, so I tried filling in a
lot of holes that I personally fell into.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
---
2010-04-10  Mike Frysinger  <vapier@gentoo.org>

	* README-HACKING: Add more sections.

 sim/README-HACKING |  245 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 245 insertions(+), 0 deletions(-)

diff --git a/sim/README-HACKING b/sim/README-HACKING
index bee504a..5219b4e 100644
--- a/sim/README-HACKING
+++ b/sim/README-HACKING
@@ -256,4 +256,249 @@ To add a new target:
 	devo/sim/<processor>/*.[ch]
 
 		Include targ-vals.h instead of syscall.h.
+
+Tracing
+=======
+
+For ports based on CGEN, tracing instrumentation should largely be for free,
+so we will cover the basic non-CGEN setup here.  The assumption is that your
+target is using the common autoconf macros and so the build system already
+includes the sim-trace configure flag.
+
+The full tracing API is covered in sim-trace.h, so this section is an overview.
+
+Before calling any trace function, you should make a call to the trace_prefix()
+function.  This is usually done in the main sim_engine_run() loop before
+simulating the next instruction.  You should make this call before every
+simulated insn.  You can probably copy & paste this:
+  if (TRACE_ANY_P (cpu))
+    trace_prefix (sd, cpu, NULL_CIA, oldpc, TRACE_LINENUM_P (cpu), NULL, 0, "");
+
+You will then need to instrument your simulator code with calls to the
+trace_generic() function with the appropriate trace index.  Typically, this
+will take a form similar to the above snippet.  So to trace instructions, you
+would use something like:
+  if (TRACE_INSN_P (cpu))
+    trace_generic (sd, cpu, TRACE_INSN_IDX, "NOP;");
+
+The exact output format is up to you.  See the trace index enum in sim-trace.h
+to see the different tracing info available.
+
+To utilize the tracing features at runtime, simply use the --trace-xxx flags.
+  run --trace-insn ./some-program
+
+Profiling
+=========
+
+Similar to the tracing section, this is merely an overview for non-CGEN based
+ports.  The full API may be found in sim-profile.h.  Its API is also similar
+to the tracing API.
+
+Note that unlike the tracing command line options, in addition to the profile
+flags, you have to use the --verbose option to view the summary report after
+execution.  Tracing output is displayed on the fly, but the profile output is
+only summarized.
+
+To profile core accesses (such as data reads/writes and insn fetches), add
+calls to PROFILE_COUNT_CORE() to your read/write functions.  So in your data
+fetch function, you'd use something like:
+  PROFILE_COUNT_CORE (cpu, target_addr, size_in_bytes, map_read);
+Then in your data write function:
+  PROFILE_COUNT_CORE (cpu, target_addr, size_in_bytes, map_write);
+And in your insn fetcher:
+  PROFILE_COUNT_CORE (cpu, target_addr, size_in_bytes, map_exec);
+
+To use the PC profiling code, you simply have to tell the system where to find
+your simulator's PC and its size.  So in your sim_open() function:
+  STATE_WATCHPOINTS (sd)->pc = address_of_cpu0_pc;
+  STATE_WATCHPOINTS (sd)->sizeof_pc = number_of_bytes_for_pc_storage;
+In a typical 32bit system, the sizeof_pc will be 4 bytes.
+
+To profile branches, in every location where a branch insn is executed, call
+one of the related helpers:
+  PROFILE_BRANCH_TAKEN (cpu);
+  PROFILE_BRANCH_UNTAKEN (cpu);
+If you have stall information, you can utilize the other helpers too.
+
+Environment Simulation
+======================
+
+The simplest simulator doesn't include environment support -- it merely
+simulates the Instruction Set Architecture (ISA).  Once you're ready to move
+on to the next level, call the common macro in your configure.ac:
+SIM_AC_OPTION_ENVIRONMENT
+
+This will support for the user, virtual, and operating environments.  See the
+sim-config.h header for a more detailed description of them.  The former are
+pretty straight forward as things like exceptions (making system calls) are
+handled in the simulator.  Which is to say, an exception does not trigger an
+exception handler in the simulator target -- that is what the operating env
+is about.  See the following userspace section for more information.
+
+Userspace System Calls
+======================
+
+By default, the libgloss userspace is simulated.  That means the system call
+numbers and calling convention matches that of libgloss.  Simulating other
+userspaces (such as Linux) is pretty straightforward, but let's first focus
+on the basics.  The basic API is covered in include/gdb/callback.h.
+
+When an instruction is simulated that invokes the system call method (such as
+forcing a hardware trap or exception), your simulator code should set up the
+CB_SYSCALL data structure before calling the common cb_syscall() function.
+For example:
+static int
+syscall_read_mem (host_callback *cb, struct cb_syscall *sc,
+		  unsigned long taddr, char *buf, int bytes)
+{
+  SIM_DESC sd = (SIM_DESC) sc->p1;
+  SIM_CPU *cpu = (SIM_CPU *) sc->p2;
+  return sim_core_read_buffer (sd, cpu, read_map, buf, taddr, bytes);
+}
+static int
+syscall_write_mem (host_callback *cb, struct cb_syscall *sc,
+		  unsigned long taddr, const char *buf, int bytes)
+{
+  SIM_DESC sd = (SIM_DESC) sc->p1;
+  SIM_CPU *cpu = (SIM_CPU *) sc->p2;
+  return sim_core_write_buffer (sd, cpu, write_map, buf, taddr, bytes);
+}
+void target_sim_syscall (SIM_CPU *cpu)
+{
+  SIM_DESC sd = CPU_STATE (cpu);
+  host_callback *cb = STATE_CALLBACK (sd);
+  CB_SYSCALL sc;
+
+  CB_SYSCALL_INIT (&sc);
+
+  sc.func = <fetch system call number>;
+  sc.arg1 = <fetch first system call argument>;
+  sc.arg2 = <fetch second system call argument>;
+  sc.arg3 = <fetch third system call argument>;
+  sc.arg4 = <fetch fourth system call argument>;
+  sc.p1 = (PTR) sd;
+  sc.p2 = (PTR) cpu;
+  sc.read_mem = syscall_read_mem;
+  sc.write_mem = syscall_write_mem;
+
+  cb_syscall (cb, &sc);
+
+  <store system call result from sc.result>;
+  <store system call error from sc.errcode>;
+}
+Some targets store the result and error code in different places, while others
+only store the error code when the result is an error.
+
+Keep in mind that the CB_SYS_xxx defines are normalized values with no real
+meaning with respect to the target.  They provide a unique map on the host so
+that it can parse things sanely.  For libgloss, the common/nltvals.def file
+creates the target's system call numbers to the CB_SYS_xxx values.
+
+To simulate other userspace targets, you really only need to update the maps
+pointers that are part of the callback interface.  So create CB_TARGET_DEFS_MAP
+arrays for each set (system calls, errnos, open bits, etc...) and in a place
+you find useful, do something like:
+
+...
+static CB_TARGET_DEFS_MAP cb_linux_syscall_map[] = {
+# define TARGET_LINUX_SYS_open 5
+  { CB_SYS_open, TARGET_LINUX_SYS_open },
+  ...
+  { -1, -1 },
+};
+...
+  host_callback *cb = STATE_CALLBACK (sd);
+  cb->syscall_map = cb_linux_syscall_map;
+  cb->errno_map = cb_linux_errno_map;
+  cb->open_map = cb_linux_open_map;
+  cb->signal_map = cb_linux_signal_map;
+  cb->stat_map = cb_linux_stat_map;
+...
+
+Each of these cb_linux_*_map's are manually declared by the arch target.
+
+The target_sim_syscall() example above will then work unchanged (ignoring the
+system call convention) because all of the callback functions go through these
+mapping arrays.
+
+Events
+======
+
+Events are scheduled and executed on behalf of either a cpu or hardware devices.
+The API is pretty much the same and can be found in common/sim-events.h and
+common/hw-events.h.
+
+For simulator targets, you really just have to worry about the schedule and
+deschedule functions.
+
+Device Trees
+============
+
+The device tree model is based on the OpenBoot specification.  Since this is
+largely inherited from the psim code, consult the existing psim documentation
+for some in-depth details.
+	http://sourceware.org/psim/manual/
+
+Hardware Devices
+================
+
+The simplest simulator doesn't include hardware device support.  Once you're
+ready to move on to the next level, call the common macro in your configure.ac:
+SIM_AC_OPTION_HARDWARE(yes,,devone devtwo devthree)
+
+The basic hardware API is documented in common/hw-device.h.
+
+Each device has to have a matching file name with a "dv-" prefix.  So there has
+to be a dv-devone.c, dv-devtwo.c, and dv-devthree.c files.  Further, each file
+has to have a matching hw_descriptor structure.  So the dv-devone.c file has to
+have something like:
+  const struct hw_descriptor dv_devone_descriptor[] = {
+    {"devone", devone_finish,},
+    {NULL, NULL},
+  };
+
+The "devone" string as well as the "devone_finish" function are not hard
+requirements, just common conventions.  The structure name is a hard
+requirement.
+
+The devone_finish() callback function is used to instantiate this device by
+parsing the corresponding properties in the device tree.
+
+Hardware devices typically attach address ranges to themselves.  Then when
+a accesses to those addresses are made, the hardware will have its callback
+invoked.  The exact callback could be a normal I/O read/write access, as
+well as a DMA access.  This makes it easy to simulate memory mapped registers.
+
+Keep in mind that like a proper device driver, it may be instantiated many
+times over.  So any device state it needs to be maintained should be allocated
+during the finish callback and attached to the hardware device via set_hw_data.
+Any hardware functions can access this private data via the hw_data function.
+
+Ports (Interrupts / IRQs)
+=========================
 
+First, a note on terminology.  A "port" is an aspect of a hardware device that
+accepts or generates interrupts.  So devices with input ports may be the target
+of an interrupt (accept it), and/or they have output ports so that they may be
+the source of an interrupt (generate it).
+
+Each port has a symbolic name and a unique number.  These are used to identify
+the port in different contexts.  The output port name has no hard relationship
+to the input port name (same for the unique number).  The callback that accepts
+the interrupt uses the name/id of its input port, while the generator function
+uses the name/id of its output port.
+
+The device tree is used to connect the output port of a device to the input
+port of another device.  There are no limits on the number of inputs connected
+to an output, or outputs to an input, or the devices attached to the ports.
+In other words, the input port and output port could be the same device.
+
+The basics are:
+ - each hardware device declares an array of ports (hw_port_descriptor).
+   any mix of input and output ports are allowed.
+ - when setting up the device, attach the array (set_hw_ports).
+ - if the device accepts interrupts, it will have to attach a port callback
+   function (set_hw_port_event)
+ - connect ports with the device tree
+ - handle incoming interrupts with the callback
+ - generate outgoing interrupts with hw_port_event
-- 
1.7.0.2