[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/kernel/kernel.module

@@ -60,6 +60,8 @@ kernel = module("kernel",
         "syscalls/system.cpp",
         "syscalls/thread.cpp",
         "syscalls/vm_area.cpp",
+        "sysconf.cpp",
+        "sysconf.h.cog",
         "task.s",
         "tss.cpp",
         "vm_space.cpp",
@@ -69,9 +71,11 @@ from glob import glob
 from os.path import join
 
 layout = join(source_root, "definitions/memory_layout.yaml")
+sysconf = join(source_root, "definitions/sysconf.yaml")
 definitions = glob('definitions/**/*.def', recursive=True)
 
 kernel.add_depends(["memory.h.cog"], [layout])
+kernel.add_depends(["sysconf.h.cog"], [sysconf])
 kernel.add_depends(["syscall.cpp.cog", "syscall.h.cog", "syscalls.inc.cog"], definitions)
 
 kernel.variables['ldflags'] = ["${ldflags}", "-T", "${source_root}/src/kernel/kernel.ld"]

src/kernel/main.cpp

@@ -27,6 +27,7 @@
 #include "scheduler.h"
 #include "serial.h"
 #include "syscall.h"
+#include "sysconf.h"
 #include "tss.h"
 #include "vm_space.h"
 
@@ -151,6 +152,7 @@ kernel_main(bootproto::args *args)
     const auto &apic_ids = devices.get_apic_ids();
     g_num_cpus = start_aps(*apic, apic_ids, args->pml4);
 
+    sysconf_create();
     interrupts_enable();
     g_com1.handle_interrupt();
 

src/kernel/memory.h.cog

@@ -55,6 +55,11 @@ for region in layout.regions:
 /// through the linear_offset area.
 constexpr bool linear_mappable(uintptr_t a) { return (a & linear_offset) == 0; }
 
+/// Get the number of pages needed to hold `bytes` bytes
+inline constexpr size_t bytes_to_pages(size_t bytes) {
+    return ((bytes - 1) / frame_size) + 1;
+}
+
 /// Convert a physical address to a virtual one (in the linear-mapped area)
 template <typename T> T * to_virtual(uintptr_t a) {
     return reinterpret_cast<T*>(a|linear_offset);

src/kernel/sysconf.cpp

@@ -0,0 +1,53 @@
+#include <string.h>
+
+#include "assert.h"
+#include "cpu.h"
+#include "frame_allocator.h"
+#include "memory.h"
+#include "sysconf.h"
+
+struct kheader
+{
+    uint64_t magic;
+    uint16_t header_len;
+    uint16_t header_version;
+
+    uint16_t version_major;
+    uint16_t version_minor;
+    uint16_t version_patch;
+    uint16_t _reserved;
+    uint32_t version_gitsha;
+
+    uint64_t flags;
+};
+
+extern kheader _kernel_header;
+
+system_config *g_sysconf = nullptr;
+uintptr_t g_sysconf_phys = 0;
+
+constexpr size_t sysconf_pages = mem::bytes_to_pages(sizeof(system_config));
+
+void
+sysconf_create()
+{
+    auto &fa = frame_allocator::get();
+
+    size_t count = fa.allocate(sysconf_pages, &g_sysconf_phys);
+
+    kassert(count == sysconf_pages,
+            "Could not get enough contiguous pages for sysconf");
+
+    g_sysconf = mem::to_virtual<system_config>(g_sysconf_phys);
+    memset(g_sysconf, 0, sysconf_pages * mem::frame_size);
+
+    g_sysconf->kernel_version_major = _kernel_header.version_major;
+    g_sysconf->kernel_version_minor = _kernel_header.version_minor;
+    g_sysconf->kernel_version_patch = _kernel_header.version_patch;
+    g_sysconf->kernel_version_gitsha = _kernel_header.version_gitsha;
+
+    g_sysconf->sys_page_size = mem::frame_size;
+    g_sysconf->sys_large_page_size = 0;
+    g_sysconf->sys_huge_page_size = 0;
+    g_sysconf->sys_num_cpus = g_num_cpus;
+}

src/kernel/sysconf.h.cog

@@ -0,0 +1,28 @@
+#pragma once
+// vim: ft=cpp
+
+/// \file sysconf.h
+/// Kernel-side implementation of sysconf struct
+
+/*[[[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_user_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]]]
+};
+
+extern system_config *g_sysconf;
+extern uintptr_t g_sysconf_phys;
+
+void sysconf_create();

src/kernel/vm_space.cpp

@@ -9,6 +9,7 @@
 #include "objects/process.h"
 #include "objects/thread.h"
 #include "objects/vm_area.h"
+#include "sysconf.h"
 #include "vm_space.h"
 
 // The initial memory for the array of areas for the kernel space
@@ -46,6 +47,14 @@ vm_space::vm_space() :
     memset(m_pml4, 0, mem::frame_size/2);
     for (unsigned i = arch::kernel_root_index; i < arch::table_entries; ++i)
         m_pml4->entries[i] = kpml4->entries[i];
+
+    // Every vm space has sysconf in it
+    vm_area *sysc = new vm_area_fixed(
+            g_sysconf_phys,
+            sizeof(system_config),
+            vm_flags::none);
+
+    add(sysconf_user_address, sysc);
 }
 
 vm_space::~vm_space()