storage: AHCI, GPT/MBR partitions, and a disk a PC can boot

q35 stopped with "no root filesystem" because its disk is SATA, like
every PC since 2008, and SchOS only had IDE. And SchFS could only sit
on a whole raw device, so nothing could boot from a disk that also
carried an ESP.

- include/blkdev.h, kernel/dev/blkdev.c: a registry of block devices
  found at probe time, numbered from 4. A disk's GPT (or MBR) is read
  when it registers, and each partition becomes a device whose block 0
  is the partition's first sector. SchFS needs no format change to live
  in a partition; a Linux SchFS driver will see the same bytes on
  /dev/sdX2. SchFS has its own GPT type:
  47138FA9-B79C-47A6-9F7D-AC1F7F572D5E.
- AHCI driver, written from the AHCI 1.3.1 spec: BIOS handoff,
  COMRESET when the link is down, IDENTIFY, 48-bit DMA read/write,
  FLUSH. Completion is polled, so the root disk does not depend on
  interrupt routing.
- USB sticks also register as usb0, so their partitions are found.
- The root probe tries registered devices between USB and IDE,
  SchFS-typed partitions first.
- make disk / tools/posix/mkdisk.sh: a GPT image with a 64 MB ESP and
  a SchFS partition, ready to dd to a stick or a disk.

Tested in QEMU q35: the old two-disk setup roots on sata1; disk.img as
the only SATA disk roots on sata0p2; disk.img as a USB stick (firmware
boots from it) roots on usb0p2. Writes survive sync in each case.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HH7xukt4aaiGj1nQEAoQf1

tools/posix/mkdisk.sh

@@ -0,0 +1,73 @@
+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0-or-later
+#
+# Build a partitioned SchOS disk: the image a real machine boots from.
+#
+#   tools/posix/mkdisk.sh <esp-dir> <schfs.img> <output.img>
+#
+# A GUID partition table with two entries:
+#
+#   1  EFI system partition   FAT, holding <esp-dir> (the loader and kernel)
+#   2  SchFS                  <schfs.img>, byte for byte, typed
+#                             47138FA9-B79C-47A6-9F7D-AC1F7F572D5E
+#
+# This is the layout every UEFI machine expects: the firmware finds the ESP by
+# its type and runs EFI/BOOT/BOOTX64.EFI from it, and the kernel finds the root
+# by scanning the same table -- the SchFS type first, anything else after (see
+# rootdev_mount() in kernel/proc/proc.c). Write it to a USB stick or a disk
+# with dd and it boots; attach it to QEMU's q35 as its only disk and it boots
+# the same way.
+#
+# The SchFS partition holds the volume unchanged. A filesystem addresses
+# blocks from the start of whatever it is on, and the kernel hands a partition
+# to it as a device whose block 0 is the partition's first sector -- so the
+# image mkschfs writes for a whole disk is already the right content for a
+# partition, and so is anything a Linux SchFS driver is given as /dev/sdX2.
+#
+# Needs sfdisk (util-linux) and mtools. Nothing here needs root or loop
+# devices: the FAT is built in a file and copied in at its offset.
+
+set -eu
+
+esp=$1
+vol=$2
+out=$3
+
+SCHFS_TYPE=47138FA9-B79C-47A6-9F7D-AC1F7F572D5E
+ESP_TYPE=C12A7328-F81F-11D2-BA4B-00A0C93EC93B
+
+for t in sfdisk mformat mcopy; do
+    command -v $t >/dev/null || { echo "mkdisk: needs $t" >&2; exit 1; }
+done
+
+# Sizes in 512-byte sectors. The ESP is 64 MB, which leaves room for the
+# kernel to grow by an order of magnitude; partitions start on 1 MB
+# boundaries, which is what every partitioning tool does and what flash
+# media erase in.
+esp_mb=64
+esp_start=2048
+esp_sectors=$((esp_mb * 2048))
+vol_bytes=$(wc -c < "$vol")
+vol_start=$((esp_start + esp_sectors))
+vol_sectors=$(( (vol_bytes + 511) / 512 ))
+# 1 MB at the end for the backup GPT, rounded the same way.
+total=$((vol_start + vol_sectors + 2048))
+
+rm -f "$out" "$out.esp"
+dd if=/dev/zero of="$out" bs=512 count=0 seek=$total 2>/dev/null
+
+sfdisk --quiet --label gpt "$out" <<EOF
+start=$esp_start, size=$esp_sectors, type=$ESP_TYPE, name="EFI system"
+start=$vol_start, size=$vol_sectors, type=$SCHFS_TYPE, name="SchOS"
+EOF
+
+# The ESP, built apart and copied in.
+dd if=/dev/zero of="$out.esp" bs=1M count=$esp_mb 2>/dev/null
+mformat -i "$out.esp" -F -v SCHOS ::
+mcopy -i "$out.esp" -s "$esp"/* ::
+dd if="$out.esp" of="$out" bs=512 seek=$esp_start conv=notrunc 2>/dev/null
+rm -f "$out.esp"
+
+dd if="$vol" of="$out" bs=512 seek=$vol_start conv=notrunc 2>/dev/null
+
+echo "mkdisk: $out -- ESP ${esp_mb} MB, SchFS $((vol_bytes >> 20)) MB"