[scripts] Add GDB j6tw page table walker

Added the command "j6tw <pml4> <addr>" which takes any arguments that
evaluate to addresses or integers. It displays the full breakdown of the
page table walk for the given address, with flags.

assets/debugging/jsix.elf-gdb.py

@@ -56,8 +56,77 @@ class PrintBacktraceCommand(gdb.Command):
                 return
 
 
+class TableWalkCommand(gdb.Command):
+    def __init__(self):
+        super().__init__("j6tw", gdb.COMMAND_DATA)
+
+    def invoke(self, arg, from_tty):
+        args = gdb.string_to_argv(arg)
+        if len(args) < 2:
+            raise Exception("Must be: j6tw <pml4> <addr>")
+
+        pml4 = int(gdb.parse_and_eval(args[0]))
+        addr = int(gdb.parse_and_eval(args[1]))
+
+        indices = [
+            (addr >> 39) & 0x1ff,
+            (addr >> 30) & 0x1ff,
+            (addr >> 21) & 0x1ff,
+            (addr >> 12) & 0x1ff,
+            ]
+
+        names = ["PML4", "PDP", "PD", "PT"]
+
+        table_flags = [
+            (0x0001, "present"),
+            (0x0002, "write"),
+            (0x0004, "user"),
+            (0x0008, "pwt"),
+            (0x0010, "pcd"),
+            (0x0020, "accessed"),
+            (0x0040, "dirty"),
+            (0x0080, "largepage"),
+            (0x0100, "global"),
+            (0x1080, "pat"),
+            ((1<<63), "xd"),
+        ]
+
+        page_flags = [
+            (0x0001, "present"),
+            (0x0002, "write"),
+            (0x0004, "user"),
+            (0x0008, "pwt"),
+            (0x0010, "pcd"),
+            (0x0020, "accessed"),
+            (0x0040, "dirty"),
+            (0x0080, "pat"),
+            (0x0100, "global"),
+            ((1<<63), "xd"),
+        ]
+
+        flagsets = [table_flags, table_flags, table_flags, page_flags]
+
+        table = pml4
+        entry = 0
+        for i in range(len(indices)):
+            entry = int(gdb.parse_and_eval(f'((uint64_t*){table})[{indices[i]}]'))
+            flagset = flagsets[i]
+            flag_names = " | ".join([f[1] for f in flagset if (entry & f[0]) == f[0]])
+
+            print(f"{names[i]:>4}: {table:016x}")
+            print(f"      index: {indices[i]:3} {entry:016x}")
+            print(f"      flags: {flag_names}")
+
+            if (entry & 1) == 0 or (i < 3 and (entry & 0x80)):
+                break
+
+            table = (entry & 0x7ffffffffffffe00) | 0xffffc00000000000
+
+
+
 PrintStackCommand()
 PrintBacktraceCommand()
+TableWalkCommand()
 
 gdb.execute("target remote :1234")
 gdb.execute("display/i $rip")