feat: aarch64 port - fix build system obstacles, add exception handling, page tables

docs/porting.md

@@ -36,7 +36,7 @@ kernel/src/Arch/
   Platform.hpp      boot-sequence hooks, platform device list, reboot
   CpuPower.hpp      frequency scaling / thermal governor (may be no-ops)
   x86_64/           the complete port
-  aarch64/          skeleton port
+  aarch64/          the port in progress (see below)
 ```
 
 Other interfaces a port implements:
@@ -85,25 +85,48 @@ are `Cpu.hpp`, `IoPort.hpp`, `PerCpu.hpp`, `Context.hpp`, `Abi.hpp` and
 - Interrupts are numbered IRQ slots (`Arch::IRQ_COUNT`); slot numbers for
   MSI-capable drivers are fixed in the drivers.
 
-## The aarch64 skeleton
+## The aarch64 port
 
 Real: CPU primitives (DAIF, WFI, `CNTVCT_EL0`, TLBI, TTBR0), per-CPU data via
 `TPIDR_EL1`, the context switch and EL0 entry (`Context.S`), TLS through
 `TPIDR_EL0`, the ELF/TLS/exit-stub ABI (syscall number in x8), the IRQ slot
-dispatch table (`ArchIrqDispatch`), panic-frame printing.
+dispatch table (`ArchIrqDispatch`), panic-frame printing, and:
+
+- Exception vectors (`Vectors.S`, installed in `EarlyInitialize`). Every
+  exception saves one 288-byte frame that is both `Arch::PanicFrame` and
+  `Arch::SyscallFrame`. EL0 `svc` goes to `SyscallDispatch`; other
+  synchronous exceptions go to `ArchSyncException` (`Exceptions.cpp`), which
+  grows user stacks and lazy anonymous memory, kills faulting processes with
+  a crash report, and panics on kernel faults.
+- Translation tables (`Paging.cpp`): 4 KiB granule, 48-bit VAs. The kernel
+  half lives in its own TTBR1 table; `KernelRoot()` is the TTBR0 table of
+  kernel contexts (EFI identity mappings only, so null pointers fault). The
+  HHDM maps only the memory map's RAM and firmware regions, never the holes
+  between them (device space on ARM). MAIR: 0 Normal WB, 1 Normal NC
+  (`MapWC`), 2 Device-nGnRE (`MapMMIO`).
 
 Not yet written (each is marked `TODO(aarch64)`):
 
-- `Paging.cpp`: translation tables and MAIR (every method panics).
-- Exception vectors (`VBAR_EL1`): synchronous faults, `svc`, IRQs.
 - GIC driver: SPI routing, SGIs for IPIs, MSIs via ITS/GICv2m
-  (`ComposeMsi` returns false, so drivers fall back to INTx/polling).
-- Generic timer tick (`Arch::Timer::InitializeBsp` returns false).
+  (`ComposeMsi` returns false, so drivers fall back to INTx/polling). IRQs
+  reach `ArchIrqException`, which only logs them.
+- Generic timer tick (`Arch::Timer::InitializeBsp` returns false). Boot
+  currently stops in the first driver that waits on `GetMilliseconds()`
+  (xHCI port power-up).
 - Secondary CPU start-up, PSCI reset and suspend.
 
-Building it needs `clang` and `ld.lld` (`make kernel ARCH=aarch64`;
-`KERNEL_LD=` points at a different `ld.lld`, e.g. rustup's
-`lib/rustlib/x86_64-unknown-linux-gnu/bin/gcc-ld/ld.lld`).
+Building it needs `clang` and `ld.lld`; without a system `ld.lld` the
+kernel makefile uses the one rustup ships
+(`~/.rustup/toolchains/*/lib/rustlib/*/bin/gcc-ld/ld.lld`), and `KERNEL_LD=`
+overrides both. `make run ARCH=aarch64` boots a kernel-only image (userspace
+is x86_64-only so far) on QEMU `virt` with a GICv2 and Cortex-A72 cores, the
+Raspberry Pi 4/400's configuration; it needs `qemu-system-aarch64` and
+`qemu-efi-aarch64`.
+
+The kernel console is mirrored to the serial port the ACPI SPCR table names
+(PL011, SBSA UART, 16550 or the BCM2835 mini UART; `Drivers/Serial/Uart.cpp`),
+including the log written before the port was found. On QEMU that is
+`-serial stdio`.
 
 ## Adding another architecture