text: /*-
   * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
   *
   * Copyright (c) 2011, Bryan Venteicher <bryanv@FreeBSD.org>
   * All rights reserved.
   *
   * Redistribution and use in source and binary forms, with or without
   * modification, are permitted provided that the following conditions
   * are met:
   * 1. Redistributions of source code must retain the above copyright
   *    notice unmodified, this list of conditions, and the following
   *    disclaimer.
   * 2. Redistributions in binary form must reproduce the above copyright
   *    notice, this list of conditions and the following disclaimer in the
   *    documentation and/or other materials provided with the distribution.
   *
   * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
   * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
   * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
   * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
   * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
   * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
   * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
   * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
   * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
   * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
   */
  
  /* Driver for VirtIO block devices. */
  
  #include <sys/cdefs.h>
  __FBSDID("$FreeBSD$");
  
  #include <sys/param.h>
  #include <sys/systm.h>
  #include <sys/kernel.h>
  #include <sys/bio.h>
  #include <sys/malloc.h>
  #include <sys/module.h>
  #include <sys/sglist.h>
  #include <sys/sysctl.h>
  #include <sys/lock.h>
  #include <sys/mutex.h>
  #include <sys/queue.h>
  
  #include <geom/geom.h>
  #include <geom/geom_disk.h>
  
  #include <machine/bus.h>
  #include <machine/resource.h>
  #include <sys/bus.h>
  #include <sys/rman.h>
  
  #include <dev/virtio/virtio.h>
  #include <dev/virtio/virtqueue.h>
  #include <dev/virtio/block/virtio_blk.h>
  
  #include "virtio_if.h"
  
  struct vtblk_request {
  	struct virtio_blk_outhdr	 vbr_hdr;
  	struct bio			*vbr_bp;
  	uint8_t				 vbr_ack;
  	TAILQ_ENTRY(vtblk_request)	 vbr_link;
  };
  
  enum vtblk_cache_mode {
  	VTBLK_CACHE_WRITETHROUGH,
  	VTBLK_CACHE_WRITEBACK,
  	VTBLK_CACHE_MAX
  };
  
  struct vtblk_softc {
  	device_t		 vtblk_dev;
  	struct mtx		 vtblk_mtx;
  	uint64_t		 vtblk_features;
  	uint32_t		 vtblk_flags;
  #define VTBLK_FLAG_INDIRECT	0x0001
  #define VTBLK_FLAG_READONLY	0x0002
  #define VTBLK_FLAG_DETACH	0x0004
  #define VTBLK_FLAG_SUSPEND	0x0008
  #define VTBLK_FLAG_BARRIER	0x0010
  #define VTBLK_FLAG_WC_CONFIG	0x0020
  
  	struct virtqueue	*vtblk_vq;
  	struct sglist		*vtblk_sglist;
  	struct disk		*vtblk_disk;
  
  	struct bio_queue_head	 vtblk_bioq;
  	TAILQ_HEAD(, vtblk_request)
  				 vtblk_req_free;
  	TAILQ_HEAD(, vtblk_request)
  				 vtblk_req_ready;
  	struct vtblk_request	*vtblk_req_ordered;
  
  	int			 vtblk_max_nsegs;
  	int			 vtblk_request_count;
  	enum vtblk_cache_mode	 vtblk_write_cache;
  
  	struct bio_queue	 vtblk_dump_queue;
  	struct vtblk_request	 vtblk_dump_request;
  };
  
  static struct virtio_feature_desc vtblk_feature_desc[] = {
  	{ VIRTIO_BLK_F_BARRIER,		"HostBarrier"	},
  	{ VIRTIO_BLK_F_SIZE_MAX,	"MaxSegSize"	},
  	{ VIRTIO_BLK_F_SEG_MAX,		"MaxNumSegs"	},
  	{ VIRTIO_BLK_F_GEOMETRY,	"DiskGeometry"	},
  	{ VIRTIO_BLK_F_RO,		"ReadOnly"	},
  	{ VIRTIO_BLK_F_BLK_SIZE,	"BlockSize"	},
  	{ VIRTIO_BLK_F_SCSI,		"SCSICmds"	},
  	{ VIRTIO_BLK_F_WCE,		"WriteCache"	},
  	{ VIRTIO_BLK_F_TOPOLOGY,	"Topology"	},
  	{ VIRTIO_BLK_F_CONFIG_WCE,	"ConfigWCE"	},
  
  	{ 0, NULL }
  };
  
  static int	vtblk_modevent(module_t, int, void *);
  
  static int	vtblk_probe(device_t);
  static int	vtblk_attach(device_t);
  static int	vtblk_detach(device_t);
  static int	vtblk_suspend(device_t);
  static int	vtblk_resume(device_t);
  static int	vtblk_shutdown(device_t);
  static int	vtblk_config_change(device_t);
  
  static int	vtblk_open(struct disk *);
  static int	vtblk_close(struct disk *);
  static int	vtblk_ioctl(struct disk *, u_long, void *, int,
  		    struct thread *);
  static int	vtblk_dump(void *, void *, vm_offset_t, off_t, size_t);
  static void	vtblk_strategy(struct bio *);
  
  static void	vtblk_negotiate_features(struct vtblk_softc *);
  static void	vtblk_setup_features(struct vtblk_softc *);
  static int	vtblk_maximum_segments(struct vtblk_softc *,
  		    struct virtio_blk_config *);
  static int	vtblk_alloc_virtqueue(struct vtblk_softc *);
  static void	vtblk_resize_disk(struct vtblk_softc *, uint64_t);
  static void	vtblk_alloc_disk(struct vtblk_softc *,
  		    struct virtio_blk_config *);
  static void	vtblk_create_disk(struct vtblk_softc *);
  
  static int	vtblk_request_prealloc(struct vtblk_softc *);
  static void	vtblk_request_free(struct vtblk_softc *);
  static struct vtblk_request *
  		vtblk_request_dequeue(struct vtblk_softc *);
  static void	vtblk_request_enqueue(struct vtblk_softc *,
  		    struct vtblk_request *);
  static struct vtblk_request *
  		vtblk_request_next_ready(struct vtblk_softc *);
  static void	vtblk_request_requeue_ready(struct vtblk_softc *,
  		    struct vtblk_request *);
  static struct vtblk_request *
  		vtblk_request_next(struct vtblk_softc *);
  static struct vtblk_request *
  		vtblk_request_bio(struct vtblk_softc *);
  static int	vtblk_request_execute(struct vtblk_softc *,
  		    struct vtblk_request *);
  static int	vtblk_request_error(struct vtblk_request *);
  
  static void	vtblk_queue_completed(struct vtblk_softc *,
  		    struct bio_queue *);
  static void	vtblk_done_completed(struct vtblk_softc *,
  		    struct bio_queue *);
  static void	vtblk_drain_vq(struct vtblk_softc *);
  static void	vtblk_drain(struct vtblk_softc *);
  
  static void	vtblk_startio(struct vtblk_softc *);
  static void	vtblk_bio_done(struct vtblk_softc *, struct bio *, int);
  
  static void	vtblk_read_config(struct vtblk_softc *,
  		    struct virtio_blk_config *);
  static void	vtblk_ident(struct vtblk_softc *);
  static int	vtblk_poll_request(struct vtblk_softc *,
  		    struct vtblk_request *);
  static int	vtblk_quiesce(struct vtblk_softc *);
  static void	vtblk_vq_intr(void *);
  static void	vtblk_stop(struct vtblk_softc *);
  
  static void	vtblk_dump_quiesce(struct vtblk_softc *);
  static int	vtblk_dump_write(struct vtblk_softc *, void *, off_t, size_t);
  static int	vtblk_dump_flush(struct vtblk_softc *);
  static void	vtblk_dump_complete(struct vtblk_softc *);
  
  static void	vtblk_set_write_cache(struct vtblk_softc *, int);
  static int	vtblk_write_cache_enabled(struct vtblk_softc *sc,
  		    struct virtio_blk_config *);
  static int	vtblk_write_cache_sysctl(SYSCTL_HANDLER_ARGS);
  
  static void	vtblk_setup_sysctl(struct vtblk_softc *);
  static int	vtblk_tunable_int(struct vtblk_softc *, const char *, int);
  
  /* Tunables. */
  static int vtblk_no_ident = 0;
  TUNABLE_INT("hw.vtblk.no_ident", &vtblk_no_ident);
  static int vtblk_writecache_mode = -1;
  TUNABLE_INT("hw.vtblk.writecache_mode", &vtblk_writecache_mode);
  
  /* Features desired/implemented by this driver. */
  #define VTBLK_FEATURES \
      (VIRTIO_BLK_F_BARRIER		| \
       VIRTIO_BLK_F_SIZE_MAX		| \
       VIRTIO_BLK_F_SEG_MAX		| \
       VIRTIO_BLK_F_GEOMETRY		| \
       VIRTIO_BLK_F_RO			| \
       VIRTIO_BLK_F_BLK_SIZE		| \
       VIRTIO_BLK_F_WCE			| \
       VIRTIO_BLK_F_TOPOLOGY		| \
       VIRTIO_BLK_F_CONFIG_WCE		| \
       VIRTIO_RING_F_INDIRECT_DESC)
  
  #define VTBLK_MTX(_sc)		&(_sc)->vtblk_mtx
  #define VTBLK_LOCK_INIT(_sc, _name) \
  				mtx_init(VTBLK_MTX((_sc)), (_name), \
  				    "VirtIO Block Lock", MTX_DEF)
  #define VTBLK_LOCK(_sc)		mtx_lock(VTBLK_MTX((_sc)))
  #define VTBLK_UNLOCK(_sc)	mtx_unlock(VTBLK_MTX((_sc)))
  #define VTBLK_LOCK_DESTROY(_sc)	mtx_destroy(VTBLK_MTX((_sc)))
  #define VTBLK_LOCK_ASSERT(_sc)	mtx_assert(VTBLK_MTX((_sc)), MA_OWNED)
  #define VTBLK_LOCK_ASSERT_NOTOWNED(_sc) \
  				mtx_assert(VTBLK_MTX((_sc)), MA_NOTOWNED)
  
  #define VTBLK_DISK_NAME		"vtbd"
  #define VTBLK_QUIESCE_TIMEOUT	(30 * hz)
  
  /*
   * Each block request uses at least two segments - one for the header
   * and one for the status.
   */
  #define VTBLK_MIN_SEGMENTS	2
  
  static device_method_t vtblk_methods[] = {
  	/* Device methods. */
  	DEVMETHOD(device_probe,		vtblk_probe),
  	DEVMETHOD(device_attach,	vtblk_attach),
  	DEVMETHOD(device_detach,	vtblk_detach),
  	DEVMETHOD(device_suspend,	vtblk_suspend),
  	DEVMETHOD(device_resume,	vtblk_resume),
  	DEVMETHOD(device_shutdown,	vtblk_shutdown),
  
  	/* VirtIO methods. */
  	DEVMETHOD(virtio_config_change,	vtblk_config_change),
  
  	DEVMETHOD_END
  };
  
  static driver_t vtblk_driver = {
  	"vtblk",
  	vtblk_methods,
  	sizeof(struct vtblk_softc)
  };
  static devclass_t vtblk_devclass;
  
  DRIVER_MODULE(virtio_blk, virtio_mmio, vtblk_driver, vtblk_devclass,
      vtblk_modevent, 0);
  DRIVER_MODULE(virtio_blk, virtio_pci, vtblk_driver, vtblk_devclass,
      vtblk_modevent, 0);
  MODULE_VERSION(virtio_blk, 1);
  MODULE_DEPEND(virtio_blk, virtio, 1, 1, 1);
  
  VIRTIO_SIMPLE_PNPTABLE(virtio_blk, VIRTIO_ID_BLOCK, "VirtIO Block Adapter");
  VIRTIO_SIMPLE_PNPINFO(virtio_mmio, virtio_blk);
  VIRTIO_SIMPLE_PNPINFO(virtio_pci, virtio_blk);
  
  static int
  vtblk_modevent(module_t mod, int type, void *unused)
  {
  	int error;
  
  	error = 0;
  
  	switch (type) {
  	case MOD_LOAD:
  	case MOD_QUIESCE:
  	case MOD_UNLOAD:
  	case MOD_SHUTDOWN:
  		break;
  	default:
  		error = EOPNOTSUPP;
  		break;
  	}
  
  	return (error);
  }
  
  static int
  vtblk_probe(device_t dev)
  {
  	return (VIRTIO_SIMPLE_PROBE(dev, virtio_blk));
  }
  
  static int
  vtblk_attach(device_t dev)
  {
  	struct vtblk_softc *sc;
  	struct virtio_blk_config blkcfg;
  	int error;
  
  	virtio_set_feature_desc(dev, vtblk_feature_desc);
  
  	sc = device_get_softc(dev);
  	sc->vtblk_dev = dev;
  	VTBLK_LOCK_INIT(sc, device_get_nameunit(dev));
  	bioq_init(&sc->vtblk_bioq);
  	TAILQ_INIT(&sc->vtblk_dump_queue);
  	TAILQ_INIT(&sc->vtblk_req_free);
  	TAILQ_INIT(&sc->vtblk_req_ready);
  
  	vtblk_setup_sysctl(sc);
  	vtblk_setup_features(sc);
  
  	vtblk_read_config(sc, &blkcfg);
  
  	/*
  	 * With the current sglist(9) implementation, it is not easy
  	 * for us to support a maximum segment size as adjacent
  	 * segments are coalesced. For now, just make sure it's larger
  	 * than the maximum supported transfer size.
  	 */
  	if (virtio_with_feature(dev, VIRTIO_BLK_F_SIZE_MAX)) {
  		if (blkcfg.size_max < MAXPHYS) {
  			error = ENOTSUP;
  			device_printf(dev, "host requires unsupported "
  			    "maximum segment size feature\n");
  			goto fail;
  		}
  	}
  
  	sc->vtblk_max_nsegs = vtblk_maximum_segments(sc, &blkcfg);
  	if (sc->vtblk_max_nsegs <= VTBLK_MIN_SEGMENTS) {
  		error = EINVAL;
  		device_printf(dev, "fewer than minimum number of segments "
  		    "allowed: %d\n", sc->vtblk_max_nsegs);
  		goto fail;
  	}
  
  	sc->vtblk_sglist = sglist_alloc(sc->vtblk_max_nsegs, M_NOWAIT);
  	if (sc->vtblk_sglist == NULL) {
  		error = ENOMEM;
  		device_printf(dev, "cannot allocate sglist\n");
  		goto fail;
  	}
  
  	error = vtblk_alloc_virtqueue(sc);
  	if (error) {
  		device_printf(dev, "cannot allocate virtqueue\n");
  		goto fail;
  	}
  
  	error = vtblk_request_prealloc(sc);
  	if (error) {
  		device_printf(dev, "cannot preallocate requests\n");
  		goto fail;
  	}
  
  	vtblk_alloc_disk(sc, &blkcfg);
  
  	error = virtio_setup_intr(dev, INTR_TYPE_BIO | INTR_ENTROPY);
  	if (error) {
  		device_printf(dev, "cannot setup virtqueue interrupt\n");
  		goto fail;
  	}
  
  	vtblk_create_disk(sc);
  
  	virtqueue_enable_intr(sc->vtblk_vq);
  
  fail:
  	if (error)
  		vtblk_detach(dev);
  
  	return (error);
  }
  
  static int
  vtblk_detach(device_t dev)
  {
  	struct vtblk_softc *sc;
  
  	sc = device_get_softc(dev);
  
  	VTBLK_LOCK(sc);
  	sc->vtblk_flags |= VTBLK_FLAG_DETACH;
  	if (device_is_attached(dev))
  		vtblk_stop(sc);
  	VTBLK_UNLOCK(sc);
  
  	vtblk_drain(sc);
  
  	if (sc->vtblk_disk != NULL) {
  		disk_destroy(sc->vtblk_disk);
  		sc->vtblk_disk = NULL;
  	}
  
  	if (sc->vtblk_sglist != NULL) {
  		sglist_free(sc->vtblk_sglist);
  		sc->vtblk_sglist = NULL;
  	}
  
  	VTBLK_LOCK_DESTROY(sc);
  
  	return (0);
  }
  
  static int
  vtblk_suspend(device_t dev)
  {
  	struct vtblk_softc *sc;
  	int error;
  
  	sc = device_get_softc(dev);
  
  	VTBLK_LOCK(sc);
  	sc->vtblk_flags |= VTBLK_FLAG_SUSPEND;
  	/* XXX BMV: virtio_stop(), etc needed here? */
  	error = vtblk_quiesce(sc);
  	if (error)
  		sc->vtblk_flags &= ~VTBLK_FLAG_SUSPEND;
  	VTBLK_UNLOCK(sc);
  
  	return (error);
  }
  
  static int
  vtblk_resume(device_t dev)
  {
  	struct vtblk_softc *sc;
  
  	sc = device_get_softc(dev);
  
  	VTBLK_LOCK(sc);
  	/* XXX BMV: virtio_reinit(), etc needed here? */
  	sc->vtblk_flags &= ~VTBLK_FLAG_SUSPEND;
  	vtblk_startio(sc);
  	VTBLK_UNLOCK(sc);
  
  	return (0);
  }
  
  static int
  vtblk_shutdown(device_t dev)
  {
  
  	return (0);
  }
  
  static int
  vtblk_config_change(device_t dev)
  {
  	struct vtblk_softc *sc;
  	struct virtio_blk_config blkcfg;
  	uint64_t capacity;
  
  	sc = device_get_softc(dev);
  
  	vtblk_read_config(sc, &blkcfg);
  
  	/* Capacity is always in 512-byte units. */
  	capacity = blkcfg.capacity * 512;
  
  	if (sc->vtblk_disk->d_mediasize != capacity)
  		vtblk_resize_disk(sc, capacity);
  
  	return (0);
  }
  
  static int
  vtblk_open(struct disk *dp)
  {
  	struct vtblk_softc *sc;
  
  	if ((sc = dp->d_drv1) == NULL)
  		return (ENXIO);
  
  	return (sc->vtblk_flags & VTBLK_FLAG_DETACH ? ENXIO : 0);
  }
  
  static int
  vtblk_close(struct disk *dp)
  {
  	struct vtblk_softc *sc;
  
  	if ((sc = dp->d_drv1) == NULL)
  		return (ENXIO);
  
  	return (0);
  }
  
  static int
  vtblk_ioctl(struct disk *dp, u_long cmd, void *addr, int flag,
      struct thread *td)
  {
  	struct vtblk_softc *sc;
  
  	if ((sc = dp->d_drv1) == NULL)
  		return (ENXIO);
  
  	return (ENOTTY);
  }
  
  static int
  vtblk_dump(void *arg, void *virtual, vm_offset_t physical, off_t offset,
      size_t length)
  {
  	struct disk *dp;
  	struct vtblk_softc *sc;
  	int error;
  
  	dp = arg;
  	error = 0;
  
  	if ((sc = dp->d_drv1) == NULL)
  		return (ENXIO);
  
  	VTBLK_LOCK(sc);
  
  	vtblk_dump_quiesce(sc);
  
  	if (length > 0)
  		error = vtblk_dump_write(sc, virtual, offset, length);
  	if (error || (virtual == NULL && offset == 0))
  		vtblk_dump_complete(sc);
  
  	VTBLK_UNLOCK(sc);
  
  	return (error);
  }
  
  static void
  vtblk_strategy(struct bio *bp)
  {
  	struct vtblk_softc *sc;
  
  	if ((sc = bp->bio_disk->d_drv1) == NULL) {
  		vtblk_bio_done(NULL, bp, EINVAL);
  		return;
  	}
  
  	/*
  	 * Fail any write if RO. Unfortunately, there does not seem to
  	 * be a better way to report our readonly'ness to GEOM above.
  	 */
  	if (sc->vtblk_flags & VTBLK_FLAG_READONLY &&
  	    (bp->bio_cmd == BIO_WRITE || bp->bio_cmd == BIO_FLUSH)) {
  		vtblk_bio_done(sc, bp, EROFS);
  		return;
  	}
  
  	if ((bp->bio_cmd != BIO_READ) && (bp->bio_cmd != BIO_WRITE) &&
  	    (bp->bio_cmd != BIO_FLUSH)) {
  		vtblk_bio_done(sc, bp, EOPNOTSUPP);
  		return;
  	}
  
  	VTBLK_LOCK(sc);
  
  	if (sc->vtblk_flags & VTBLK_FLAG_DETACH) {
  		VTBLK_UNLOCK(sc);
  		vtblk_bio_done(sc, bp, ENXIO);
  		return;
  	}
  
  	bioq_insert_tail(&sc->vtblk_bioq, bp);
  	vtblk_startio(sc);
  
  	VTBLK_UNLOCK(sc);
  }
  
  static void
  vtblk_negotiate_features(struct vtblk_softc *sc)
  {
  	device_t dev;
  	uint64_t features;
  
  	dev = sc->vtblk_dev;
  	features = VTBLK_FEATURES;
  
  	sc->vtblk_features = virtio_negotiate_features(dev, features);
  }
  
  static void
  vtblk_setup_features(struct vtblk_softc *sc)
  {
  	device_t dev;
  
  	dev = sc->vtblk_dev;
  
  	vtblk_negotiate_features(sc);
  
  	if (virtio_with_feature(dev, VIRTIO_RING_F_INDIRECT_DESC))
  		sc->vtblk_flags |= VTBLK_FLAG_INDIRECT;
  	if (virtio_with_feature(dev, VIRTIO_BLK_F_RO))
  		sc->vtblk_flags |= VTBLK_FLAG_READONLY;
  	if (virtio_with_feature(dev, VIRTIO_BLK_F_BARRIER))
  		sc->vtblk_flags |= VTBLK_FLAG_BARRIER;
  	if (virtio_with_feature(dev, VIRTIO_BLK_F_CONFIG_WCE))
  		sc->vtblk_flags |= VTBLK_FLAG_WC_CONFIG;
  }
  
  static int
  vtblk_maximum_segments(struct vtblk_softc *sc,
      struct virtio_blk_config *blkcfg)
  {
  	device_t dev;
  	int nsegs;
  
  	dev = sc->vtblk_dev;
  	nsegs = VTBLK_MIN_SEGMENTS;
  
  	if (virtio_with_feature(dev, VIRTIO_BLK_F_SEG_MAX)) {
  		nsegs += MIN(blkcfg->seg_max, MAXPHYS / PAGE_SIZE + 1);
  		if (sc->vtblk_flags & VTBLK_FLAG_INDIRECT)
  			nsegs = MIN(nsegs, VIRTIO_MAX_INDIRECT);
  	} else
  		nsegs += 1;
  
  	return (nsegs);
  }
  
  static int
  vtblk_alloc_virtqueue(struct vtblk_softc *sc)
  {
  	device_t dev;
  	struct vq_alloc_info vq_info;
  
  	dev = sc->vtblk_dev;
  
  	VQ_ALLOC_INFO_INIT(&vq_info, sc->vtblk_max_nsegs,
  	    vtblk_vq_intr, sc, &sc->vtblk_vq,
  	    "%s request", device_get_nameunit(dev));
  
  	return (virtio_alloc_virtqueues(dev, 0, 1, &vq_info));
  }
  
  static void
  vtblk_resize_disk(struct vtblk_softc *sc, uint64_t new_capacity)
  {
  	device_t dev;
  	struct disk *dp;
  	int error;
  
  	dev = sc->vtblk_dev;
  	dp = sc->vtblk_disk;
  
  	dp->d_mediasize = new_capacity;
  	if (bootverbose) {
  		device_printf(dev, "resized to %juMB (%ju %u byte sectors)\n",
  		    (uintmax_t) dp->d_mediasize >> 20,
  		    (uintmax_t) dp->d_mediasize / dp->d_sectorsize,
  		    dp->d_sectorsize);
  	}
  
  	error = disk_resize(dp, M_NOWAIT);
  	if (error) {
  		device_printf(dev,
  		    "disk_resize(9) failed, error: %d\n", error);
  	}
  }
  
  static void
  vtblk_alloc_disk(struct vtblk_softc *sc, struct virtio_blk_config *blkcfg)
  {
  	device_t dev;
  	struct disk *dp;
  
  	dev = sc->vtblk_dev;
  
  	sc->vtblk_disk = dp = disk_alloc();
  	dp->d_open = vtblk_open;
  	dp->d_close = vtblk_close;
  	dp->d_ioctl = vtblk_ioctl;
  	dp->d_strategy = vtblk_strategy;
  	dp->d_name = VTBLK_DISK_NAME;
  	dp->d_unit = device_get_unit(dev);
  	dp->d_drv1 = sc;
  	dp->d_flags = DISKFLAG_CANFLUSHCACHE | DISKFLAG_UNMAPPED_BIO |
  	    DISKFLAG_DIRECT_COMPLETION;
  	dp->d_hba_vendor = virtio_get_vendor(dev);
  	dp->d_hba_device = virtio_get_device(dev);
  	dp->d_hba_subvendor = virtio_get_subvendor(dev);
  	dp->d_hba_subdevice = virtio_get_subdevice(dev);
  
  	if ((sc->vtblk_flags & VTBLK_FLAG_READONLY) == 0)
  		dp->d_dump = vtblk_dump;
  
  	/* Capacity is always in 512-byte units. */
  	dp->d_mediasize = blkcfg->capacity * 512;
  
  	if (virtio_with_feature(dev, VIRTIO_BLK_F_BLK_SIZE))
  		dp->d_sectorsize = blkcfg->blk_size;
  	else
  		dp->d_sectorsize = 512;
  
  	/*
  	 * The VirtIO maximum I/O size is given in terms of segments.
  	 * However, FreeBSD limits I/O size by logical buffer size, not
  	 * by physically contiguous pages. Therefore, we have to assume
  	 * no pages are contiguous. This may impose an artificially low
  	 * maximum I/O size. But in practice, since QEMU advertises 128
  	 * segments, this gives us a maximum IO size of 125 * PAGE_SIZE,
  	 * which is typically greater than MAXPHYS. Eventually we should
  	 * just advertise MAXPHYS and split buffers that are too big.
  	 *
  	 * Note we must subtract one additional segment in case of non
  	 * page aligned buffers.
  	 */
  	dp->d_maxsize = (sc->vtblk_max_nsegs - VTBLK_MIN_SEGMENTS - 1) *
  	    PAGE_SIZE;
  	if (dp->d_maxsize < PAGE_SIZE)
  		dp->d_maxsize = PAGE_SIZE; /* XXX */
  
  	if (virtio_with_feature(dev, VIRTIO_BLK_F_GEOMETRY)) {
  		dp->d_fwsectors = blkcfg->geometry.sectors;
  		dp->d_fwheads = blkcfg->geometry.heads;
  	}
  
  	if (virtio_with_feature(dev, VIRTIO_BLK_F_TOPOLOGY) &&
  	    blkcfg->topology.physical_block_exp > 0) {
  		dp->d_stripesize = dp->d_sectorsize *
  		    (1 << blkcfg->topology.physical_block_exp);
  		dp->d_stripeoffset = (dp->d_stripesize -
  		    blkcfg->topology.alignment_offset * dp->d_sectorsize) %
  		    dp->d_stripesize;
  	}
  
  	if (vtblk_write_cache_enabled(sc, blkcfg) != 0)
  		sc->vtblk_write_cache = VTBLK_CACHE_WRITEBACK;
  	else
  		sc->vtblk_write_cache = VTBLK_CACHE_WRITETHROUGH;
  }
  
  static void
  vtblk_create_disk(struct vtblk_softc *sc)
  {
  	struct disk *dp;
  
  	dp = sc->vtblk_disk;
  
  	vtblk_ident(sc);
  
  	device_printf(sc->vtblk_dev, "%juMB (%ju %u byte sectors)\n",
  	    (uintmax_t) dp->d_mediasize >> 20,
  	    (uintmax_t) dp->d_mediasize / dp->d_sectorsize,
  	    dp->d_sectorsize);
  
  	disk_create(dp, DISK_VERSION);
  }
  
  static int
  vtblk_request_prealloc(struct vtblk_softc *sc)
  {
  	struct vtblk_request *req;
  	int i, nreqs;
  
  	nreqs = virtqueue_size(sc->vtblk_vq);
  
  	/*
  	 * Preallocate sufficient requests to keep the virtqueue full. Each
  	 * request consumes VTBLK_MIN_SEGMENTS or more descriptors so reduce
  	 * the number allocated when indirect descriptors are not available.
  	 */
  	if ((sc->vtblk_flags & VTBLK_FLAG_INDIRECT) == 0)
  		nreqs /= VTBLK_MIN_SEGMENTS;
  
  	for (i = 0; i < nreqs; i++) {
  		req = malloc(sizeof(struct vtblk_request), M_DEVBUF, M_NOWAIT);
  		if (req == NULL)
  			return (ENOMEM);
  
  		MPASS(sglist_count(&req->vbr_hdr, sizeof(req->vbr_hdr)) == 1);
  		MPASS(sglist_count(&req->vbr_ack, sizeof(req->vbr_ack)) == 1);
  
  		sc->vtblk_request_count++;
  		vtblk_request_enqueue(sc, req);
  	}
  
  	return (0);
  }
  
  static void
  vtblk_request_free(struct vtblk_softc *sc)
  {
  	struct vtblk_request *req;
  
  	MPASS(TAILQ_EMPTY(&sc->vtblk_req_ready));
  
  	while ((req = vtblk_request_dequeue(sc)) != NULL) {
  		sc->vtblk_request_count--;
  		free(req, M_DEVBUF);
  	}
  
  	KASSERT(sc->vtblk_request_count == 0,
  	    ("%s: leaked %d requests", __func__, sc->vtblk_request_count));
  }
  
  static struct vtblk_request *
  vtblk_request_dequeue(struct vtblk_softc *sc)
  {
  	struct vtblk_request *req;
  
  	req = TAILQ_FIRST(&sc->vtblk_req_free);
  	if (req != NULL) {
  		TAILQ_REMOVE(&sc->vtblk_req_free, req, vbr_link);
  		bzero(req, sizeof(struct vtblk_request));
  	}
  
  	return (req);
  }
  
  static void
  vtblk_request_enqueue(struct vtblk_softc *sc, struct vtblk_request *req)
  {
  
  	TAILQ_INSERT_HEAD(&sc->vtblk_req_free, req, vbr_link);
  }
  
  static struct vtblk_request *
  vtblk_request_next_ready(struct vtblk_softc *sc)
  {
  	struct vtblk_request *req;
  
  	req = TAILQ_FIRST(&sc->vtblk_req_ready);
  	if (req != NULL)
  		TAILQ_REMOVE(&sc->vtblk_req_ready, req, vbr_link);
  
  	return (req);
  }
  
  static void
  vtblk_request_requeue_ready(struct vtblk_softc *sc, struct vtblk_request *req)
  {
  
  	/* NOTE: Currently, there will be at most one request in the queue. */
  	TAILQ_INSERT_HEAD(&sc->vtblk_req_ready, req, vbr_link);
  }
  
  static struct vtblk_request *
  vtblk_request_next(struct vtblk_softc *sc)
  {
  	struct vtblk_request *req;
  
  	req = vtblk_request_next_ready(sc);
  	if (req != NULL)
  		return (req);
  
  	return (vtblk_request_bio(sc));
  }
  
  static struct vtblk_request *
  vtblk_request_bio(struct vtblk_softc *sc)
  {
  	struct bio_queue_head *bioq;
  	struct vtblk_request *req;
  	struct bio *bp;
  
  	bioq = &sc->vtblk_bioq;
  
  	if (bioq_first(bioq) == NULL)
  		return (NULL);
  
  	req = vtblk_request_dequeue(sc);
  	if (req == NULL)
  		return (NULL);
  
  	bp = bioq_takefirst(bioq);
  	req->vbr_bp = bp;
  	req->vbr_ack = -1;
  	req->vbr_hdr.ioprio = 1;
  
  	switch (bp->bio_cmd) {
  	case BIO_FLUSH:
  		req->vbr_hdr.type = VIRTIO_BLK_T_FLUSH;
  		break;
  	case BIO_READ:
  		req->vbr_hdr.type = VIRTIO_BLK_T_IN;
  		req->vbr_hdr.sector = bp->bio_offset / 512;
  		break;
  	case BIO_WRITE:
  		req->vbr_hdr.type = VIRTIO_BLK_T_OUT;
  		req->vbr_hdr.sector = bp->bio_offset / 512;
  		break;
  	default:
  		panic("%s: bio with unhandled cmd: %d", __func__, bp->bio_cmd);
  	}
  
  	if (bp->bio_flags & BIO_ORDERED)
  		req->vbr_hdr.type |= VIRTIO_BLK_T_BARRIER;
  
  	return (req);
  }
  
  static int
  vtblk_request_execute(struct vtblk_softc *sc, struct vtblk_request *req)
  {
  	struct virtqueue *vq;
  	struct sglist *sg;
  	struct bio *bp;
  	int ordered, readable, writable, error;
  
  	vq = sc->vtblk_vq;
  	sg = sc->vtblk_sglist;
  	bp = req->vbr_bp;
  	ordered = 0;
  	writable = 0;
  
  	/*
  	 * Some hosts (such as bhyve) do not implement the barrier feature,
  	 * so we emulate it in the driver by allowing the barrier request
  	 * to be the only one in flight.
  	 */
  	if ((sc->vtblk_flags & VTBLK_FLAG_BARRIER) == 0) {
  		if (sc->vtblk_req_ordered != NULL)
  			return (EBUSY);
  		if (bp->bio_flags & BIO_ORDERED) {
  			if (!virtqueue_empty(vq))
  				return (EBUSY);
  			ordered = 1;
  			req->vbr_hdr.type &= ~VIRTIO_BLK_T_BARRIER;
  		}
  	}
  
  	sglist_reset(sg);
  	sglist_append(sg, &req->vbr_hdr, sizeof(struct virtio_blk_outhdr));
  
  	if (bp->bio_cmd == BIO_READ || bp->bio_cmd == BIO_WRITE) {
  		error = sglist_append_bio(sg, bp);
  		if (error || sg->sg_nseg == sg->sg_maxseg) {
  			panic("%s: bio %p data buffer too big %d",
  			    __func__, bp, error);
  		}
  
  		/* BIO_READ means the host writes into our buffer. */
  		if (bp->bio_cmd == BIO_READ)
  			writable = sg->sg_nseg - 1;
  	}
  
  	writable++;
  	sglist_append(sg, &req->vbr_ack, sizeof(uint8_t));
  	readable = sg->sg_nseg - writable;
  
  	error = virtqueue_enqueue(vq, req, sg, readable, writable);
  	if (error == 0 && ordered)
  		sc->vtblk_req_ordered = req;
  
  	return (error);
  }
  
  static int
  vtblk_request_error(struct vtblk_request *req)
  {
  	int error;
  
  	switch (req->vbr_ack) {
  	case VIRTIO_BLK_S_OK:
  		error = 0;
  		break;
  	case VIRTIO_BLK_S_UNSUPP:
  		error = ENOTSUP;
  		break;
  	default:
  		error = EIO;
  		break;
  	}
  
  	return (error);
  }
  
  static void
  vtblk_queue_completed(struct vtblk_softc *sc, struct bio_queue *queue)
  {
  	struct vtblk_request *req;
  	struct bio *bp;
  
  	while ((req = virtqueue_dequeue(sc->vtblk_vq, NULL)) != NULL) {
  		if (sc->vtblk_req_ordered != NULL) {
  			MPASS(sc->vtblk_req_ordered == req);
  			sc->vtblk_req_ordered = NULL;
  		}
  
  		bp = req->vbr_bp;
  		bp->bio_error = vtblk_request_error(req);
  		TAILQ_INSERT_TAIL(queue, bp, bio_queue);
  
  		vtblk_request_enqueue(sc, req);
  	}
  }
  
  static void
  vtblk_done_completed(struct vtblk_softc *sc, struct bio_queue *queue)
  {
  	struct bio *bp, *tmp;
  
  	TAILQ_FOREACH_SAFE(bp, queue, bio_queue, tmp) {
  		if (bp->bio_error != 0)
  			disk_err(bp, "hard error", -1, 1);
  		vtblk_bio_done(sc, bp, bp->bio_error);
  	}
  }
  
  static void
  vtblk_drain_vq(struct vtblk_softc *sc)
  {
  	struct virtqueue *vq;
  	struct vtblk_request *req;
  	int last;
  
  	vq = sc->vtblk_vq;
  	last = 0;
  
  	while ((req = virtqueue_drain(vq, &last)) != NULL) {
  		vtblk_bio_done(sc, req->vbr_bp, ENXIO);
  		vtblk_request_enqueue(sc, req);
  	}
  
  	sc->vtblk_req_ordered = NULL;
  	KASSERT(virtqueue_empty(vq), ("virtqueue not empty"));
  }
  
  static void
  vtblk_drain(struct vtblk_softc *sc)
  {
  	struct bio_queue queue;
  	struct bio_queue_head *bioq;
  	struct vtblk_request *req;
  	struct bio *bp;
  
  	bioq = &sc->vtblk_bioq;
  	TAILQ_INIT(&queue);
  
  	if (sc->vtblk_vq != NULL) {
  		vtblk_queue_completed(sc, &queue);
  		vtblk_done_completed(sc, &queue);
  
  		vtblk_drain_vq(sc);
  	}
  
  	while ((req = vtblk_request_next_ready(sc)) != NULL) {
  		vtblk_bio_done(sc, req->vbr_bp, ENXIO);
  		vtblk_request_enqueue(sc, req);
  	}
  
  	while (bioq_first(bioq) != NULL) {
  		bp = bioq_takefirst(bioq);
  		vtblk_bio_done(sc, bp, ENXIO);
  	}
  
  	vtblk_request_free(sc);
  }
  
  static void
  vtblk_startio(struct vtblk_softc *sc)
  {
  	struct virtqueue *vq;
  	struct vtblk_request *req;
  	int enq;
  
  	VTBLK_LOCK_ASSERT(sc);
  	vq = sc->vtblk_vq;
  	enq = 0;
  
  	if (sc->vtblk_flags & VTBLK_FLAG_SUSPEND)
  		return;
  
  	while (!virtqueue_full(vq)) {
  		req = vtblk_request_next(sc);
  		if (req == NULL)
  			break;
  
  		if (vtblk_request_execute(sc, req) != 0) {
  			vtblk_request_requeue_ready(sc, req);
  			break;
  		}
  
  		enq++;
  	}
  
  	if (enq > 0)
  		virtqueue_notify(vq);
  }
  
  static void
  vtblk_bio_done(struct vtblk_softc *sc, struct bio *bp, int error)
  {
  
  	/* Because of GEOM direct dispatch, we cannot hold any locks. */
  	if (sc != NULL)
  		VTBLK_LOCK_ASSERT_NOTOWNED(sc);
  
  	if (error) {
  		bp->bio_resid = bp->bio_bcount;
  		bp->bio_error = error;
  		bp->bio_flags |= BIO_ERROR;
  	}
  
  	biodone(bp);
  }
  
  #define VTBLK_GET_CONFIG(_dev, _feature, _field, _cfg)			\
  	if (virtio_with_feature(_dev, _feature)) {			\
  		virtio_read_device_config(_dev,				\
  		    offsetof(struct virtio_blk_config, _field),		\
  		    &(_cfg)->_field, sizeof((_cfg)->_field));		\
  	}
  
  static void
  vtblk_read_config(struct vtblk_softc *sc, struct virtio_blk_config *blkcfg)
  {
  	device_t dev;
  
  	dev = sc->vtblk_dev;
  
  	bzero(blkcfg, sizeof(struct virtio_blk_config));
  
  	/* The capacity is always available. */
  	virtio_read_device_config(dev, offsetof(struct virtio_blk_config,
  	    capacity), &blkcfg->capacity, sizeof(blkcfg->capacity));
  
  	/* Read the configuration if the feature was negotiated. */
  	VTBLK_GET_CONFIG(dev, VIRTIO_BLK_F_SIZE_MAX, size_max, blkcfg);
  	VTBLK_GET_CONFIG(dev, VIRTIO_BLK_F_SEG_MAX, seg_max, blkcfg);
  	VTBLK_GET_CONFIG(dev, VIRTIO_BLK_F_GEOMETRY, geometry, blkcfg);
  	VTBLK_GET_CONFIG(dev, VIRTIO_BLK_F_BLK_SIZE, blk_size, blkcfg);
  	VTBLK_GET_CONFIG(dev, VIRTIO_BLK_F_TOPOLOGY, topology, blkcfg);
  	VTBLK_GET_CONFIG(dev, VIRTIO_BLK_F_CONFIG_WCE, writeback, blkcfg);
  }
  
  #undef VTBLK_GET_CONFIG
  
  static void
  vtblk_ident(struct vtblk_softc *sc)
  {
  	struct bio buf;
  	struct disk *dp;
  	struct vtblk_request *req;
  	int len, error;
  
  	dp = sc->vtblk_disk;
  	len = MIN(VIRTIO_BLK_ID_BYTES, DISK_IDENT_SIZE);
  
  	if (vtblk_tunable_int(sc, "no_ident", vtblk_no_ident) != 0)
  		return;
  
  	req = vtblk_request_dequeue(sc);
  	if (req == NULL)
  		return;
  
  	req->vbr_ack = -1;
  	req->vbr_hdr.type = VIRTIO_BLK_T_GET_ID;
  	req->vbr_hdr.ioprio = 1;
  	req->vbr_hdr.sector = 0;
  
  	req->vbr_bp = &buf;
  	g_reset_bio(&buf);
  
  	buf.bio_cmd = BIO_READ;
  	buf.bio_data = dp->d_ident;
  	buf.bio_bcount = len;
  
  	VTBLK_LOCK(sc);
  	error = vtblk_poll_request(sc, req);
  	VTBLK_UNLOCK(sc);
  
  	vtblk_request_enqueue(sc, req);
  
  	if (error) {
  		device_printf(sc->vtblk_dev,
  		    "error getting device identifier: %d\n", error);
  	}
  }
  
  static int
  vtblk_poll_request(struct vtblk_softc *sc, struct vtblk_request *req)
  {
  	struct virtqueue *vq;
  	int error;
  
  	vq = sc->vtblk_vq;
  
  	if (!virtqueue_empty(vq))
  		return (EBUSY);
  
  	error = vtblk_request_execute(sc, req);
  	if (error)
  		return (error);
  
  	virtqueue_notify(vq);
  	virtqueue_poll(vq, NULL);
  
  	error = vtblk_request_error(req);
  	if (error && bootverbose) {
  		device_printf(sc->vtblk_dev,
  		    "%s: IO error: %d\n", __func__, error);
  	}
  
  	return (error);
  }
  
  static int
  vtblk_quiesce(struct vtblk_softc *sc)
  {
  	int error;
  
  	VTBLK_LOCK_ASSERT(sc);
  	error = 0;
  
  	while (!virtqueue_empty(sc->vtblk_vq)) {
  		if (mtx_sleep(&sc->vtblk_vq, VTBLK_MTX(sc), PRIBIO, "vtblkq",
  		    VTBLK_QUIESCE_TIMEOUT) == EWOULDBLOCK) {
  			error = EBUSY;
  			break;
  		}
  	}
  
  	return (error);
  }
  
  static void
  vtblk_vq_intr(void *xsc)
  {
  	struct vtblk_softc *sc;
  	struct virtqueue *vq;
  	struct bio_queue queue;
  
  	sc = xsc;
  	vq = sc->vtblk_vq;
  	TAILQ_INIT(&queue);
  
  	VTBLK_LOCK(sc);
  
  again:
  	if (sc->vtblk_flags & VTBLK_FLAG_DETACH)
  		goto out;
  
  	vtblk_queue_completed(sc, &queue);
  	vtblk_startio(sc);
  
  	if (virtqueue_enable_intr(vq) != 0) {
  		virtqueue_disable_intr(vq);
  		goto again;
  	}
  
  	if (sc->vtblk_flags & VTBLK_FLAG_SUSPEND)
  		wakeup(&sc->vtblk_vq);
  
  out:
  	VTBLK_UNLOCK(sc);
  	vtblk_done_completed(sc, &queue);
  }
  
  static void
  vtblk_stop(struct vtblk_softc *sc)
  {
  
  	virtqueue_disable_intr(sc->vtblk_vq);
  	virtio_stop(sc->vtblk_dev);
  }
  
  static void
  vtblk_dump_quiesce(struct vtblk_softc *sc)
  {
  
  	/*
  	 * Spin here until all the requests in-flight at the time of the
  	 * dump are completed and queued. The queued requests will be
  	 * biodone'd once the dump is finished.
  	 */
  	while (!virtqueue_empty(sc->vtblk_vq))
  		vtblk_queue_completed(sc, &sc->vtblk_dump_queue);
  }
  
  static int
  vtblk_dump_write(struct vtblk_softc *sc, void *virtual, off_t offset,
      size_t length)
  {
  	struct bio buf;
  	struct vtblk_request *req;
  
  	req = &sc->vtblk_dump_request;
  	req->vbr_ack = -1;
  	req->vbr_hdr.type = VIRTIO_BLK_T_OUT;
  	req->vbr_hdr.ioprio = 1;
  	req->vbr_hdr.sector = offset / 512;
  
  	req->vbr_bp = &buf;
  	g_reset_bio(&buf);
  
  	buf.bio_cmd = BIO_WRITE;
  	buf.bio_data = virtual;
  	buf.bio_bcount = length;
  
  	return (vtblk_poll_request(sc, req));
  }
  
  static int
  vtblk_dump_flush(struct vtblk_softc *sc)
  {
  	struct bio buf;
  	struct vtblk_request *req;
  
  	req = &sc->vtblk_dump_request;
  	req->vbr_ack = -1;
  	req->vbr_hdr.type = VIRTIO_BLK_T_FLUSH;
  	req->vbr_hdr.ioprio = 1;
  	req->vbr_hdr.sector = 0;
  
  	req->vbr_bp = &buf;
  	g_reset_bio(&buf);
  
  	buf.bio_cmd = BIO_FLUSH;
  
  	return (vtblk_poll_request(sc, req));
  }
  
  static void
  vtblk_dump_complete(struct vtblk_softc *sc)
  {
  
  	vtblk_dump_flush(sc);
  
  	VTBLK_UNLOCK(sc);
  	vtblk_done_completed(sc, &sc->vtblk_dump_queue);
  	VTBLK_LOCK(sc);
  }
  
  static void
  vtblk_set_write_cache(struct vtblk_softc *sc, int wc)
  {
  
  	/* Set either writeback (1) or writethrough (0) mode. */
  	virtio_write_dev_config_1(sc->vtblk_dev,
  	    offsetof(struct virtio_blk_config, writeback), wc);
  }
  
  static int
  vtblk_write_cache_enabled(struct vtblk_softc *sc,
      struct virtio_blk_config *blkcfg)
  {
  	int wc;
  
  	if (sc->vtblk_flags & VTBLK_FLAG_WC_CONFIG) {
  		wc = vtblk_tunable_int(sc, "writecache_mode",
  		    vtblk_writecache_mode);
  		if (wc >= 0 && wc < VTBLK_CACHE_MAX)
  			vtblk_set_write_cache(sc, wc);
  		else
  			wc = blkcfg->writeback;
  	} else
  		wc = virtio_with_feature(sc->vtblk_dev, VIRTIO_BLK_F_WCE);
  
  	return (wc);
  }
  
  static int
  vtblk_write_cache_sysctl(SYSCTL_HANDLER_ARGS)
  {
  	struct vtblk_softc *sc;
  	int wc, error;
  
  	sc = oidp->oid_arg1;
  	wc = sc->vtblk_write_cache;
  
  	error = sysctl_handle_int(oidp, &wc, 0, req);
  	if (error || req->newptr == NULL)
  		return (error);
  	if ((sc->vtblk_flags & VTBLK_FLAG_WC_CONFIG) == 0)
  		return (EPERM);
  	if (wc < 0 || wc >= VTBLK_CACHE_MAX)
  		return (EINVAL);
  
  	VTBLK_LOCK(sc);
  	sc->vtblk_write_cache = wc;
  	vtblk_set_write_cache(sc, sc->vtblk_write_cache);
  	VTBLK_UNLOCK(sc);
  
  	return (0);
  }
  
  static void
  vtblk_setup_sysctl(struct vtblk_softc *sc)
  {
  	device_t dev;
  	struct sysctl_ctx_list *ctx;
  	struct sysctl_oid *tree;
  	struct sysctl_oid_list *child;
  
  	dev = sc->vtblk_dev;
  	ctx = device_get_sysctl_ctx(dev);
  	tree = device_get_sysctl_tree(dev);
  	child = SYSCTL_CHILDREN(tree);
  
  	SYSCTL_ADD_PROC(ctx, child, OID_AUTO, "writecache_mode",
  	    CTLTYPE_INT | CTLFLAG_RW | CTLFLAG_MPSAFE, sc, 0,
  	    vtblk_write_cache_sysctl, "I",
  	    "Write cache mode (writethrough (0) or writeback (1))");
  }
  
  static int
  vtblk_tunable_int(struct vtblk_softc *sc, const char *knob, int def)
  {
  	char path[64];
  
  	snprintf(path, sizeof(path),
  	    "hw.vtblk.%d.%s", device_get_unit(sc->vtblk_dev), knob);
  	TUNABLE_INT_FETCH(path, &def);
  
  	return (def);
  }
  