ldls: the top-2 GB window Linux modules link against

A Linux module is built with -mcmodel=kernel and can only reach addresses
in the top 2 GB, where this kernel is not linked. So the kernel image is
mapped a second time at LDLS_KALIAS, with a vmemmap area and a module area
beside it, in one PDPT shared by every address space. Nothing loads a
module yet; this is the address-space groundwork, set up at every boot,
and include/ldls/ is the interface the loader will use.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4

include/ldls/ldls.h

@@ -0,0 +1,53 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+#pragma once
+
+// /dev/ldls -- Linux drivers, in the kernel. docs/architecture/ldls.md.
+//
+// The device is a control channel and nothing else: a module is handed over
+// whole, as bytes, together with the BTF of the kernel it was built for, and
+// the kernel does the rest -- loading, linking, init_module, probe, and
+// bringing the interface up -- on a thread of its own. What happened comes
+// back as text, because the useful answer to "did it work" is the driver's
+// own account of what it found.
+//
+//     ldls load /lib/modules/e1000.ko
+//     ldls list
+//     ldls unload e1000
+//
+// Shared with userland (cmd/ldls), so it is spelled in plain C types.
+
+#define LDLS_MAJOR 13
+
+// Take the device even if a kernel driver holds it, provided that driver can
+// be asked to let go. Without this LDLS only takes devices nothing else
+// drives, which is the right default at boot: the kernel's own driver is on
+// the path to a shell, and the hosted one is the better one only when someone
+// says so.
+#define LDLS_TAKE     0x1
+// Load and link, run init_module, and stop: no device is probed. For a
+// library module (mii, libphy) whose job is to export symbols to the next.
+#define LDLS_NOPROBE  0x2
+
+struct ldls_load {
+  unsigned long long ko;        // in: the module image
+  unsigned long long kolen;
+  unsigned long long btf;       // in: the kernel's BTF, /sys/kernel/btf/vmlinux
+  unsigned long long btflen;
+  unsigned long long msg;       // out: what happened, NUL-terminated
+  unsigned int       msgcap;
+  unsigned int       flags;     // LDLS_*
+  int                bus, slot, func;   // -1 for "whichever matches"
+  unsigned int       hz;        // CONFIG_HZ of that kernel; 0 means 1000
+  int                id;        // out: the module's id, for unload
+};
+
+struct ldls_text {
+  unsigned long long buf;       // out
+  unsigned int       cap;
+  unsigned int       pad;
+};
+
+#define LDLSIOC_LOAD    0xD000  // struct ldls_load
+#define LDLSIOC_UNLOAD  0xD001  // in: int id
+#define LDLSIOC_LIST    0xD002  // struct ldls_text
+#define LDLSIOC_LOG     0xD003  // struct ldls_text, in: pad = id