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/arch/memlayout.h

@@ -91,6 +91,22 @@ extern uint64 lowtop;
 #define KERNPHYS 0x1000000
 #define KERNLINK (KERNBASE+KERNPHYS) // Address where kernel is linked
 
+// The LDLS window, in the top 2 GB, where a Linux module expects to live. See
+// ldls_vm_init() in kernel/arch/x86_64/vm.c.
+//
+//   LDLS_VMEMMAP  1 GB   Linux's `struct page` array, 64 bytes per frame --
+//                        enough for 64 GB, not for all of PHYSTOP_MAX; a
+//                        frame above that has no struct page. Backed on demand.
+//   LDLS_KALIAS   512 MB the kernel image again, from physical zero
+//   LDLS_MODBASE  512 MB modules, their stacks and their vmalloc
+#define LDLS_VMEMMAP 0xFFFFFFFF40000000ULL
+#define LDLS_KALIAS  0xFFFFFFFF80000000ULL
+#define LDLS_MODBASE 0xFFFFFFFFA0000000ULL
+#define LDLS_MODEND  0xFFFFFFFFC0000000ULL
+
+// A kernel address, as a module sees it.
+#define LDLS_ALIAS(a) ((uint64)(a) - KERNBASE + LDLS_KALIAS)
+
 #ifndef __ASSEMBLER__
 static inline addr_t v2p(void *a) {
   return ((addr_t)(a)) - ((addr_t)KERNBASE);

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);

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