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/kldls.h

@@ -0,0 +1,26 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+#pragma once
+
+// The kernel's side of LDLS, as the rest of the kernel sees it. The layer
+// itself is kernel/ldls/; this is the handful of places it is called from.
+
+struct trapframe;
+
+// kernel/arch/x86_64/vm.c -- the window in the top 2 GB.
+void    ldls_vm_init(void);
+int     kmap_page(uint64 va, addr_t pa);
+addr_t  kmap_phys(uint64 va);
+addr_t  kunmap_page(uint64 va);
+
+// kernel/ldls/core.c
+void    ldls_init(void);
+int     ldls_ioctl(uint64 request, uint64 argp);
+
+// A trap in kernel mode. Returns 1 if LDLS dealt with it -- a WARN() that was
+// reported and stepped over, or a hosted driver that faulted and has been
+// stopped -- and 0 if it is the kernel's own and should panic as before.
+int     ldls_trap(struct trapframe *tf);
+
+// The name of an address in a hosted module, for a backtrace. 0 if `a` is not
+// in one.
+int     ldls_symbolize(uint64 a, char *out, int cap);