ahci: SATA driver + FAT32 persistent disk mounted at /disk

- ahci.rs: polling AHCI on PCI class 1/6/1, bus mastering + ABAR map,
  port 0 up, IDENTIFY + READ/WRITE DMA EXT through one 4KiB DMA buffer
- fat.rs: BPB parse, whole-FAT RAM cache with write-back to both copies,
  LFN read/write, dirs, multi-cluster files, FSInfo free-count sync,
  space-padded dot entries ('..' = cluster 0 under root), fsck-clean
- fs.rs: /disk mount node + 24-slot ephemeral node pool; ls/cat/hexdump/
  echo/mkdir/touch/rm/edit all work on disk paths unchanged
- QEMU: explicit bus assignment (disk=ide.0, ESP=ide.1); the implicit
  fat:rw:esp drive was stealing AHCI port 0
- Debugged against QEMU source: header DW0 is CFL in bits 4:0 / PRDTL in
  bits 31:16 (Linux libahci layout), FIS count at bytes 12-13, device at
  byte 7; QEMU rejects H2D FIS flags bits 6:4, so direction is the
  header W bit only
- Verified: full shell round-trip on /disk, reboot persistence (poweroff
  then mdir/mtype and a fresh boot), fsck.fat clean

1 files changed, 15 insertions, 3 deletions

diff --git a/Makefile b/Makefile
index 1d60062..7701d9d 100644
--- a/Makefile
+++ b/Makefile
@@ -10,6 +10,9 @@ export INITRD=$(ESP_DIR)/initramfs.cpio.gz
 export BOOTLOADER=rsboot/rsboot.efi
 export OVMF=/usr/share/edk2/x64/OVMF.4m.fd
 export BOOT_EFI=$(ESP_DIR)/EFI/BOOT/BOOTX64.EFI
+# Attach a raw disk image as the SATA drive (disk.img -> ide.0, ESP -> ide.1).
+# Create one with: mkfs.fat -F 32 -n RSDOS disk.img && mcopy -i disk.img files ::
+export DISK_IMG ?= /tmp/disk.img
 
 .PHONY: all run run-gui clean image debug release fmt bootloader boot-image
 
@@ -43,7 +46,10 @@ $(INITRD): | $(ESP_DIR)
 run: all
         qemu-system-x86_64 \
           -bios $(OVMF) \
-          -drive file=fat:rw:$(ESP_DIR),format=raw \
+          -drive file=$(DISK_IMG),format=raw,if=none,id=disk \
+          -device ide-hd,drive=disk,bus=ide.0 \
+          -drive file=fat:rw:$(ESP_DIR),format=raw,if=none,id=esp \
+          -device ide-hd,drive=esp,bus=ide.1 \
           -serial stdio \
           -m 256M \
           -machine q35 \
@@ -54,7 +60,10 @@ run: all
 run-gui: all
         qemu-system-x86_64 \
           -bios $(OVMF) \
-          -drive file=fat:rw:$(ESP_DIR),format=raw \
+          -drive file=$(DISK_IMG),format=raw,if=none,id=disk \
+          -device ide-hd,drive=disk,bus=ide.0 \
+          -drive file=fat:rw:$(ESP_DIR),format=raw,if=none,id=esp \
+          -device ide-hd,drive=esp,bus=ide.1 \
           -serial stdio \
           -m 256M \
           -machine q35 \
@@ -66,7 +75,10 @@ run-gui: all
 debug: all
         qemu-system-x86_64 \
           -bios $(OVMF) \
-          -drive file=fat:rw:$(ESP_DIR),format=raw \
+          -drive file=$(DISK_IMG),format=raw,if=none,id=disk \
+          -device ide-hd,drive=disk,bus=ide.0 \
+          -drive file=fat:rw:$(ESP_DIR),format=raw,if=none,id=esp \
+          -device ide-hd,drive=esp,bus=ide.1 \
           -serial stdio \
           -m 256M \
           -machine q35 \