Demand paging, the RTL8168H, links that come up late, and Ctrl+Alt+F1..F12

- Anonymous memory can be reserved without being backed: large mappings,
  MAP_NORESERVE and PROT_NONE are recorded and each page appears, zeroed,
  on first touch -- from trap() for the program, from uva2ka() for the
  kernel on its behalf. WebKit reserves 4 GB at start and touches a sliver;
  backing it all failed with ENOMEM and Epiphany aborted.
- r8169: the 8168G generation and later (the laptop's 8168H, XID 541) are
  taken from the firmware's out-of-band mode, get their PLL powered, the
  receive gate opened, and their PHY reached through the OCP window. The
  PHY's power-down bit is cleared; the old code kept it while restarting
  autonegotiation. Untested on the hardware so far.
- The network thread follows each device's link into lwIP. The link was
  read once at attach, 0.02 s into boot; real autonegotiation takes
  seconds, so on hardware the interface stayed down for good.
- Twelve terminals, switched with Ctrl+Alt+F1..F12 by default: the keys X
  uses, so the console and the desktop agree. AltGr+F-key is gone from
  the defaults (chvt -c altgr+F brings it back).
- SO_PASSCRED on Unix sockets, with SCM_CREDENTIALS delivered by recvmsg.
  WebKit sets it on its IPC sockets and aborted when refused.

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

kernel/arch/x86_64/vm.c

@@ -1134,6 +1134,13 @@ copyuvm(pml4e_t *pgdir, uint64 sz)
   return 0;
 }
 
+// The PTE for va, if its page tables exist. For vm_fault().
+pte_t *
+walkpte(pml4e_t *pgdir, const void *va)
+{
+  return walkpgdir(pgdir, va, 0);
+}
+
 char*
 uva2ka(pml4e_t *pgdir, char *uva)
 {
@@ -1142,6 +1149,11 @@ uva2ka(pml4e_t *pgdir, char *uva)
 
   pte_t *pte;
   pte = walkpgdir(pgdir, uva, 0);
+  // Anonymous memory not touched yet: backed now, as the program's own
+  // access would have done. Only for the running process's own table.
+  if((pte == 0 || (*pte & PTE_P) == 0) && myproc() && pgdir == myproc()->pgdir &&
+     vm_fault((addr_t)uva) == 0)
+    pte = walkpgdir(pgdir, uva, 0);
   if(pte == 0 || (*pte & PTE_P) == 0)
     return 0;
   if((*pte & PTE_U) == 0)