userland/freestanding/gem5_* work on baremetal

Remove all the duplicates of those present throughout the tree.

README.adoc

@@ -11321,13 +11321,13 @@ since boot has already happened, and the parameters are already in the RAM of th
 
 ==== gem5 checkpoint userland minimal example
 
-In order to debug checkpoint restore bugs, this minimal setup using link:userland/freestanding/gem5_checkpoint_restore.S[] can be handy:
+In order to debug checkpoint restore bugs, this minimal setup using link:userland/freestanding/gem5_checkpoint.S[] can be handy:
 
 ....
 ./build-userland --arch aarch64 --static
-./run --arch aarch64 --emulator gem5 --static --userland userland/freestanding/gem5_checkpoint_restore.S --trace-insts-stdout
+./run --arch aarch64 --emulator gem5 --static --userland userland/freestanding/gem5_checkpoint.S --trace-insts-stdout
-./run --arch aarch64 --emulator gem5 --static --userland userland/freestanding/gem5_checkpoint_restore.S --trace-insts-stdout --gem5-restore 1
+./run --arch aarch64 --emulator gem5 --static --userland userland/freestanding/gem5_checkpoint.S --trace-insts-stdout --gem5-restore 1
-./run --arch aarch64 --emulator gem5 --static --userland userland/freestanding/gem5_checkpoint_restore.S --trace-insts-stdout --gem5-restore 1 -- --cpu-type=DerivO3CPU --restore-with-cpu=DerivO3CPU --caches
+./run --arch aarch64 --emulator gem5 --static --userland userland/freestanding/gem5_checkpoint.S --trace-insts-stdout --gem5-restore 1 -- --cpu-type=DerivO3CPU --restore-with-cpu=DerivO3CPU --caches
 ....
 
 On the initial run, we see that all instructions are executed and the checkpoint is taken:
@@ -11512,7 +11512,7 @@ And then restore the checkpoint with a different slower CPU:
 
 And now you will notice that everything happens much slower in the guest terminal!
 
-One even more direct and minimal way to observe this is with link:userland/freestanding/gem5_checkpoint_restore.S[] which was mentioned at <<gem5-checkpoint-userland-minimal-example>> plus some logging:
+One even more direct and minimal way to observe this is with link:userland/freestanding/gem5_checkpoint.S[] which was mentioned at <<gem5-checkpoint-userland-minimal-example>> plus some logging:
 
 ....
 ./run \
@@ -11520,7 +11520,7 @@ One even more direct and minimal way to observe this is with link:userland/frees
   --emulator gem5 \
   --static \
   --trace ExecAll,FmtFlag,O3CPU,SimpleCPU \
-  --userland userland/freestanding/gem5_checkpoint_restore.S \
+  --userland userland/freestanding/gem5_checkpoint.S \
 ;
 cat "$(./getvar --arch aarch64 --emulator gem5 trace_txt_file)"
 ./run \
@@ -11529,7 +11529,7 @@ cat "$(./getvar --arch aarch64 --emulator gem5 trace_txt_file)"
   --gem5-restore 1 \
   --static \
   --trace ExecAll,FmtFlag,O3CPU,SimpleCPU \
-  --userland userland/freestanding/gem5_checkpoint_restore.S \
+  --userland userland/freestanding/gem5_checkpoint.S \
   -- \
   --caches \
   --cpu-type DerivO3CPU \
@@ -11584,7 +11584,7 @@ This is generally useless compared to checkpoint restoring because:
 * checkpoint restore allows to run multiple contents after the restore, and restoring to multiple different system states, which you almost always want to do
 * we generally don't know the exact tick at which the region of interest will start, especially as the binaries change. It is much easier to just instrument the content with a checkoint <<m5ops,m5op>>
 
-But let's give it a try anyways with link:userland/freestanding/gem5_checkpoint_restore.S[] which was mentioned at <<gem5-checkpoint-userland-minimal-example>>
+But let's give it a try anyways with link:userland/freestanding/gem5_checkpoint.S[] which was mentioned at <<gem5-checkpoint-userland-minimal-example>>
 
 ....
 ./run \
@@ -11592,7 +11592,7 @@ But let's give it a try anyways with link:userland/freestanding/gem5_checkpoint_
   --emulator gem5 \
   --static \
   --trace ExecAll,FmtFlag,O3CPU,SimpleCPU \
-  --userland userland/freestanding/gem5_checkpoint_restore.S \
+  --userland userland/freestanding/gem5_checkpoint.S \
   -- \
   --caches
   --cpu-type DerivO3CPU \
@@ -17003,6 +17003,7 @@ The C standard library infrastructure is implemented in the common userland / ba
 
 Unlike most our other assembly examples, which use the C standard library for portability, examples under `freestanding/` directories don't link to the C standard library:
 
+* link:userland/freestanding/[]: freestanding programs that work on any ISA
 * link:userland/arch/x86_64/freestanding/[]
 * link:userland/arch/arm/freestanding/[]
 * link:userland/arch/aarch64/freestanding/[]

baremetal/arch/aarch64/no_bootloader/gem5_exit.S

@@ -1,5 +0,0 @@
-#include <lkmc/m5ops.h>
-
-.global _start
-_start:
-    LKMC_M5OPS_EXIT_ASM

baremetal/arch/arm/no_bootloader/gem5_exit.S

@@ -1,3 +0,0 @@
-.global _start
-_start:
-    mov r0, #0; mov r1, #0; .inst 0xEE000110 | (0x21 << 16);

baremetal/lib/syscalls_asm.S

@@ -7,6 +7,7 @@
  * void _exit(int status)
  *
  * If only there was a GCC attribute to create such a function!
+ * https://stackoverflow.com/questions/43310704/creating-a-c-function-without-compiler-generated-prologue-epilogue-ret-instruc
  */
 .text
 .global _exit

common.py

@@ -1716,10 +1716,16 @@ after configure, e.g. SCons. Usually contains specific targets or other build fl
             if extra_objs is None:
                 extra_objs= []
             if link:
-                if self.env['mode'] == 'baremetal' or my_path_properties['extra_objs_lkmc_common']:
+                #  Baremetal builds cannot add their usual syscall objects, as those
+                # rely on standard library symbols.
+                if my_path_properties['freestanding']:
+                    extra_objs = []
+                if (self.env['mode'] == 'baremetal' and not my_path_properties['freestanding']) \
+                        or my_path_properties['extra_objs_lkmc_common']:
                     extra_objs.extend(extra_objs_lkmc_common)
                 if (
                     self.env['mode'] == 'baremetal' and
+                    not my_path_properties['freestanding'] and
                     not my_path_properties['extra_objs_disable_baremetal_bootloader']
                 ):
                     extra_objs.extend(extra_objs_baremetal_bootloader)

path_properties.py

@@ -50,6 +50,7 @@ class PathProperties:
         'extra_objs_disable_baremetal_bootloader': False,
         # We should get rid of this if we ever properly implement dependency graphs.
         'extra_objs_lkmc_common': False,
+        'freestanding': True,
         'gem5_unimplemented_instruction': False,
         # Fully, or partially unimplemented.
         'gem5_unimplemented_syscall': False,
@@ -374,13 +375,18 @@ freestanding_properties = {
         '-static', LF,
     ],
     'extra_objs_lkmc_common': False,
+    'freestanding': True,
 }
 # https://cirosantilli.com/linux-kernel-module-cheat#nostartfiles-programs
 nostartfiles_properties = {
+    # The baremetal bootloader sets up the stack to a valid value.
+    # Therefore, without it, C code may not be called, and so this is very restrictive in general.
+    # Programs that don't call C code nor use stack can still work.
     'baremetal': False,
     'cc_flags': [
         '-nostartfiles', LF,
     ],
+    'extra_objs_disable_baremetal_bootloader': True,
 }
 # See: https://cirosantilli.com/linux-kernel-module-cheat#path-properties
 path_properties_tuples = (
@@ -406,7 +412,6 @@ path_properties_tuples = (
                                 'no_bootloader': (
                                     {'extra_objs_disable_baremetal_bootloader': True},
                                     {
-                                        'gem5_exit.S': {'requires_m5ops': True},
                                         'multicore_asm.S': {'test_run_args': {'cpus': 2}},
                                         'semihost_exit.S': {'requires_semihosting': True},
                                     }
@@ -423,7 +428,6 @@ path_properties_tuples = (
                                 'no_bootloader': (
                                     {'extra_objs_disable_baremetal_bootloader': True},
                                     {
-                                        'gem5_exit.S': {'requires_m5ops': True},
                                         'multicore_asm.S': {'test_run_args': {'cpus': 2}},
                                         'semihost_exit.S': {'requires_semihosting': True},
                                         'wfe_loop.S': {'more_than_1s': True},
@@ -685,9 +689,9 @@ path_properties_tuples = (
                     },
                 ),
                 'freestanding': (
-                    freestanding_properties,
+                    {**freestanding_properties, **{'baremetal': True}},
                     {
-                        'gem5_checkpoint_restore.S': {'allowed_emulators': {'gem5'}},
+                        'gem5_checkpoint.S': {'allowed_emulators': {'gem5'}},
                         'gem5_exit.S': {'allowed_emulators': {'gem5'}},
                     }
                 ),

userland/arch/aarch64/freestanding/linux/gem5_exit.S

@@ -1,10 +0,0 @@
-/* https://cirosantilli.com/linux-kernel-module-cheat#benchmark-emulators-on-userland-executables */
-
-#define LKMC_M5OPS_ENABLE 1
-#include "lkmc/m5ops.h"
-
-.text
-.global _start
-_start:
-asm_main_after_prologue:
-    LKMC_M5OPS_EXIT_ASM

userland/freestanding/README.adoc

@@ -0,0 +1 @@
+https://cirosantilli.com/linux-kernel-module-cheat#freestanding-programs