[kernel] Expose a sysconf page to userspace

A structure, system_config, which is dynamically defined by the
definitions/sysconf.yaml config, is now mapped into every user address
space. The kernel fills this with information about itself and the
running machine.

User programs access this through the new j6_sysconf fake syscall in
libj6.

See: Github bug #242
See: [frobozz blog post](https://jsix.dev/posts/frobozz/)

Tags:

src/libraries/j6/include/j6/sysconf.h.cog

@@ -0,0 +1,44 @@
+#pragma once
+// vim: ft=cpp
+
+/// \file sysconf.h
+/// Get kernel configuration
+
+// The kernel depends on libj6 for some shared code,
+// but should not include the user-specific code.
+#ifndef __j6kernel
+
+#include <stddef.h>
+#include <stdint.h>
+
+/*[[[cog code generation
+from os.path import join
+from sysconf import Sysconf
+sc = Sysconf(join(definitions_path, "sysconf.yaml"))
+]]]*/
+///[[[end]]]
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+enum j6_sysconf_arg
+{
+    /*[[[cog code generation
+    for var in sc.vars:
+        cog.outl(f"j6sc_{var.name},")
+    ]]]*/
+    ///[[[end]]]
+
+    j6sc_MAX
+};
+
+/// Get the kernel configuration value specified by
+/// the argument.
+unsigned long j6_sysconf(j6_sysconf_arg arg);
+
+#ifdef __cplusplus
+} // extern C
+#endif
+
+#endif // __j6kernel

src/libraries/j6/j6.module

@@ -6,10 +6,15 @@ j6 = module("j6",
     sources = [
         "init.cpp",
         "include/j6/syscalls.h.cog",
+        "include/j6/sysconf.h.cog",
         "syscalls.s.cog",
+        "sysconf.cpp.cog",
     ])
 
 from glob import glob
+from os.path import join
+
+sysconf = join(source_root, "definitions/sysconf.yaml")
 definitions = glob('definitions/**/*.def', recursive=True)
 
 j6.add_depends([
@@ -17,3 +22,8 @@ j6.add_depends([
         "syscalls.s.cog",
     ], definitions)
 
+j6.add_depends([
+        "include/j6/sysconf.h.cog",
+        "sysconf.cpp.cog",
+    ], [sysconf])
+

src/libraries/j6/sysconf.cpp.cog

@@ -0,0 +1,44 @@
+// vim: ft=cpp
+
+// The kernel depends on libj6 for some shared code,
+// but should not include the user-specific code.
+#ifndef __j6kernel
+
+#include <j6/sysconf.h>
+
+/*[[[cog code generation
+from os.path import join
+from sysconf import Sysconf
+sc = Sysconf(join(definitions_path, "sysconf.yaml"))
+
+cog.outl(f"constexpr uintptr_t __sysconf_address = {sc.address:#x};")
+]]]*/
+///[[[end]]]
+
+struct __system_config
+{
+    /*[[[cog code generation
+    for var in sc.vars:
+        cog.outl(f"{var.type:10} {var.section}_{var.name};")
+    ]]]*/
+    ///[[[end]]]
+};
+
+unsigned long
+j6_sysconf(j6_sysconf_arg arg)
+{
+    __system_config &sc =
+       * reinterpret_cast<__system_config*>(__sysconf_address);
+
+    switch (arg) {
+    /*[[[cog code generation
+    for var in sc.vars:
+        cog.outl(f"case j6sc_{var.name}: return sc.{var.section}_{var.name};")
+    ]]]*/
+    ///[[[end]]]
+
+    default: return 0;
+    }
+}
+
+#endif // __j6kernel