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, 4 insertions, 1 deletions

diff --git a/scripts/qemu.sh b/scripts/qemu.sh
index b75bed4..123d9d0 100755
--- a/scripts/qemu.sh
+++ b/scripts/qemu.sh
@@ -40,7 +40,10 @@ while [ $# -gt 0 ]; do
                 ;;
         --image | -i)
                 _img_file="$(printf -- '-drive file=%s,format=raw,if=none,id=hd' "$2")"
-                DRIVE_ARGS=("$_img_file" '-device' 'ide-hd,drive=hd')
+                # Explicit buses: data disk -> ide.0 (port 0), ESP -> ide.1.
+                DRIVE_ARGS=("$_img_file" '-device' 'ide-hd,drive=hd,bus=ide.0' \
+                        '-drive' 'file=fat:rw:esp,format=raw,if=none,id=esp' \
+                        '-device' 'ide-hd,drive=esp,bus=ide.1')
                 shift 2
                 ;;
         --bios | -b)