Fix CR3 handling in interrupt/syscall paths using per-CPU data

[ryanbreen]

Summary

• Fix hardcoded kernel CR3 (0x101000) that became stale after master PML4 creation
• Add kernel_cr3 and saved_process_cr3 fields to PerCpuData struct
• Interrupt/syscall entry now reads kernel CR3 from gs:[72] instead of hardcoded value
• Process CR3 saved on entry at gs:[80], restored on exit when no context switch
• Fix SyscallFrame struct field order to match assembly push order (was reversed)
• Fix syscall_return_to_userspace to clear next_cr3 BEFORE CR3 switch (not after)

Root Cause

The kernel was triple faulting because:

1. per_cpu::init() stored the bootloader's CR3 (0x101000) as kernel_cr3
2. build_master_kernel_pml4() created a new master PML4 at 0x552000
3. Assembly code used hardcoded 0x101000, causing page faults when switching CR3

Test plan

• cargo run -p xtask -- boot-stages passes 36/40 stages
• Zero compilation warnings
• Userspace syscalls work (Stage 38: "Hello from userspace!")
• ENOSYS handling works (Stage 39: "ENOSYS OK")
• Multiple timer interrupts preempt userspace without crashes
• CR3 debug markers ($CR3, !CR3) show correct switching

🤖 Generated with Claude Code

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Note

Use per-CPU kernel_cr3/saved_process_cr3 for CR3 switching/restoration in interrupts/syscalls, update kernel_cr3 after master PML4, fix SyscallFrame layout, and clear next_cr3 before CR3 switch.

• Interrupts/Syscalls (assembly):
  • Swap to kernel GS early and save process CR3 to gs:[80]; read kernel CR3 from gs:[72] (removes hardcoded 0x101000).
  • On return, clear next_cr3 at gs:[64] before switching CR3; if no context switch, restore saved process CR3; ensure proper swapgs sequencing.
  • Apply same logic to first userspace entry and syscall paths; add lightweight serial markers for diagnostics.
• Per-CPU data (kernel/src/per_cpu.rs):
  • Extend PerCpuData with kernel_cr3 (offset 72) and saved_process_cr3 (offset 80) plus constants; update size/asserts.
  • Initialize kernel_cr3 from current CR3 in init(); add get_kernel_cr3/set_kernel_cr3 accessors.
• Memory init (kernel/src/memory/mod.rs):
  • After building master PML4, read current CR3 and update per-CPU kernel_cr3.
• Syscall frame (kernel/src/syscall/handler.rs):
  • Reorder SyscallFrame fields to match push order (r15 at RSP+0 … rax at RSP+112).

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