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/drivers/usb/storage/usb_storage.c

@@ -25,6 +25,7 @@
 #include "buf.h"
 #include "dma.h"
 #include "usb.h"
+#include "blkdev.h"
 
 #define CBW_SIGNATURE  0x43425355       // "USBC"
 #define CSW_SIGNATURE  0x53425355       // "USBS"
@@ -332,6 +333,40 @@ usb_storage_dump(void)
   brelse(b);
 }
 
+// The same disk, as the block device registry sees it: whole, as "usb0", and
+// with its partitions read and registered behind it. A stick written with
+// tools/posix/mkdisk.sh -- or by any installer -- keeps SchFS in a partition
+// next to its ESP, and the fixed USBDISK_DEV, which is the whole medium, can
+// never see it there.
+static int
+ums_blk_rw(void *priv, uint64 lba, uint32 count, void *buf, int write)
+{
+  int n;
+  (void)priv;
+  if(count * SECTOR_SIZE > BSIZE || lba + count > ums.nsectors)
+    return -1;
+  acquiresleep(&ums.lock);
+  if(write)
+    memmove(ums.xfer, buf, count * SECTOR_SIZE);
+  n = scsi_rw((uint32)lba, count, ums.xfer, write);
+  if(n == (int)(count * SECTOR_SIZE) && !write)
+    memmove(buf, ums.xfer, count * SECTOR_SIZE);
+  releasesleep(&ums.lock);
+  return n == (int)(count * SECTOR_SIZE) ? 0 : -1;
+}
+
+static int
+ums_blk_flush(void *priv)
+{
+  (void)priv;
+  return usb_storage_flush();
+}
+
+static const struct blkdev_ops ums_blk_ops = {
+  .rw    = ums_blk_rw,
+  .flush = ums_blk_flush,
+};
+
 // ---------------------------------------------------------------------------
 
 static int
@@ -407,6 +442,7 @@ storage_probe(struct usb_dev *dev)
   cprintf("usb-storage: %s %s, %d sectors (%d MB)\n",
           vendor, product, ums.nsectors,
           (int)((uint64)ums.nsectors * SECTOR_SIZE / (1024 * 1024)));
+  blkdev_register("usb0", &ums_blk_ops, 0, ums.nsectors, SECTOR_SIZE);
   return 0;
 
 fail: