[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/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;
+}