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

kernel/proc/proc.c

@@ -14,6 +14,7 @@
 #include "file.h"
 #include "posix.h"
 #include "errno.h"
+#include "blkdev.h"
 
 // Verify myproc()->fs_base offset matches the hardcoded 0x688 in trapasm.S.
 _Static_assert(__builtin_offsetof(struct proc, fs_base) == 0x18,
@@ -884,6 +885,8 @@ rootdev_usable(uint dev)
     return ramdisk_present();
   if (dev == USBDISK_DEV)
     return usb_storage_present();
+  if (dev >= BLKDEV_FIRST)
+    return blkdev_present(dev);
   // iderw() panics on an absent drive rather than failing, so a probe has to
   // ask first. That is the whole reason ide_disk_present() exists.
   return ide_disk_present((int)dev);
@@ -892,16 +895,27 @@ rootdev_usable(uint dev)
 static void
 rootdev_mount(void)
 {
-  static const struct { uint dev; const char *what; } cand[] = {
+  // The fixed devices, with the registered ones -- SATA, NVMe, and every
+  // partition on them -- between USB and IDE. A partition GPT types as SchFS
+  // goes ahead of the rest of the registry, because it was labelled that way
+  // on purpose; the others are probed too, so a volume in an untyped or MBR
+  // partition, or on a whole unpartitioned disk, still boots.
+  struct { uint dev; const char *what; } cand[4 + 2 * NBLKDEV];
+  int ncand = 0;
 #ifdef RAMDISK_ROOT
-    { RAMDISK_DEV, "ramdisk"    },
+  cand[ncand].dev = RAMDISK_DEV; cand[ncand++].what = "ramdisk";
 #endif
-    { USBDISK_DEV, "USB disk"   },
-    { 1,           "IDE disk 1" },
-    { 0,           "IDE disk 0" },
-  };
+  cand[ncand].dev = USBDISK_DEV; cand[ncand++].what = "USB disk";
+  for (int pass = 0; pass < 2; pass++)
+    for (int i = 0, d; (d = blkdev_nth(i)) >= 0; i++)
+      if (blkdev_is_schfs(d) == !pass) {
+        cand[ncand].dev = d;
+        cand[ncand++].what = blkdev_name(d);
+      }
+  cand[ncand].dev = 1; cand[ncand++].what = "IDE disk 1";
+  cand[ncand].dev = 0; cand[ncand++].what = "IDE disk 0";
 
-  for (int i = 0; i < (int)NELEM(cand); i++) {
+  for (int i = 0; i < ncand; i++) {
     if (!rootdev_usable(cand[i].dev))
       continue;
     schfs_kmount(cand[i].dev);
@@ -922,7 +936,7 @@ rootdev_mount(void)
   // which is how the first version of this message came out as
   // `%-11s device 1596400`. See print_int() in kernel/core/console.c.
   cprintf("root: no SchFS volume on any candidate device. Looked at:\n");
-  for (int i = 0; i < (int)NELEM(cand); i++)
+  for (int i = 0; i < ncand; i++)
     cprintf("  device %d, %s: %s\n", cand[i].dev, cand[i].what,
             rootdev_usable(cand[i].dev) ? "present, but no SchFS on it"
                                         : "not present");