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

Makefile

@@ -24,7 +24,7 @@ KERN_CORE = $(addprefix $(BUILD)/kernel/core/, \
               panic.o uaccess.o qr.o time.o ksym.o klog.o sbuf.o kmem.o)
 
 KERN_DEV  = $(addprefix $(BUILD)/kernel/dev/, \
-              bio.o cdev.o device.o disk.o fb.o ide.o input.o irq.o kbd.o keymap.o mouse.o \
+              bio.o blkdev.o cdev.o device.o disk.o fb.o ide.o input.o irq.o kbd.o keymap.o mouse.o \
               pci.o pciuser.o text.o ttf.o uart.o gfx.o vt.o ramdisk.o procfs.o \
               dbg.o netuser.o audio.o)
 
@@ -47,6 +47,7 @@ KERN_DRIVERS += $(BUILD)/kernel/drivers/net/r8169/r8169.o
 KERN_DRIVERS += $(BUILD)/kernel/drivers/net/rtl8139/rtl8139.o
 KERN_DRIVERS += $(BUILD)/kernel/drivers/audio/ac97.o
 KERN_DRIVERS += $(BUILD)/kernel/drivers/bus/xhci/xhci.o
+KERN_DRIVERS += $(BUILD)/kernel/drivers/storage/ahci/ahci.o
 KERN_DRIVERS += $(BUILD)/kernel/drivers/usb/core/usb.o
 KERN_DRIVERS += $(BUILD)/kernel/drivers/usb/hid/usb_hid.o
 KERN_DRIVERS += $(BUILD)/kernel/drivers/usb/storage/usb_storage.o
@@ -736,6 +737,13 @@ iso:
 schos.iso: $(BUILD)/kernel/kernel $(BUILD)/uefi/BOOTX64.EFI sysroot-install
 	SYSROOT=$(SYSROOT) sh tools/posix/mkiso.sh $@
 
+# A partitioned disk -- GPT, an ESP and a SchFS partition -- which is what a
+# real machine boots from: dd it to a USB stick or a disk. See
+# tools/posix/mkdisk.sh. `make qemu-disk` boots it on q35 as the only disk.
+disk: $(BUILD)/disk.img
+$(BUILD)/disk.img: esp $(BUILD)/schfs.img tools/posix/mkdisk.sh
+	sh tools/posix/mkdisk.sh $(BUILD)/esp $(BUILD)/schfs.img $@
+
 # Boot the ISO the way a machine booting off a disc would: no hard disk at
 # all, so a kernel that still secretly depends on one fails here rather than
 # in someone's optical drive.
@@ -1505,7 +1513,7 @@ qrtest: $(QRTEST)
 qrcheck: $(QRTEST)
 	$(PYTHON) tools/python/qrcheck.py
 
-.PHONY: all kernel ramroot cmd ports projects sysroot-install qemu qemu-console qemu-kvm schcom-test \
+.PHONY: disk all kernel ramroot cmd ports projects sysroot-install qemu qemu-console qemu-kvm schcom-test \
         schfs-test schfs-objtest schfs-image-test schfs-check kmem-test mkschfs \
         schfs-fsck crashshot qemu-schfs iso qemu-iso clean distclean \
         logo qrtest qrcheck crashqr usb-ids ovmf esp uefi