MFC r343585: Only sort requests of types that have concept of offset.

Other types, such as BIO_FLUSH or BIO_ZONE, or especially new/unknown ones,
may imply some degree of ordering even if strict ordering is not requested
explicitly.

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Correct bioq_disksort so that bioq_insert_tail() offers barrier semantic.
Add the BIO_ORDERED flag for struct bio and update bio clients to use it.

The barrier semantics of bioq_insert_tail() were broken in two ways:

o In bioq_disksort(), an added bio could be inserted at the head of
the queue, even when a barrier was present, if the sort key for
the new entry was less than that of the last queued barrier bio.

o The last_offset used to generate the sort key for newly queued bios
did not stay at the position of the barrier until either the
barrier was de-queued, or a new barrier (which updates last_offset)
was queued. When a barrier is in effect, we know that the disk
will pass through the barrier position just before the
"blocked bios" are released, so using the barrier's offset for
last_offset is the optimal choice.

sys/geom/sched/subr_disk.c:
sys/kern/subr_disk.c:
o Update last_offset in bioq_insert_tail().

o Only update last_offset in bioq_remove() if the removed bio is
at the head of the queue (typically due to a call via
bioq_takefirst()) and no barrier is active.

o In bioq_disksort(), if we have a barrier (insert_point is non-NULL),
set prev to the barrier and cur to it's next element. Now that
last_offset is kept at the barrier position, this change isn't
strictly necessary, but since we have to take a decision branch
anyway, it does avoid one, no-op, loop iteration in the while
loop that immediately follows.

o In bioq_disksort(), bypass the normal sort for bios with the
BIO_ORDERED attribute and instead insert them into the queue
with bioq_insert_tail(). bioq_insert_tail() not only gives
the desired command order during insertion, but also provides
barrier semantics so that commands disksorted in the future
cannot pass the just enqueued transaction.

sys/sys/bio.h:
Add BIO_ORDERED as bit 4 of the bio_flags field in struct bio.

sys/cam/ata/ata_da.c:
sys/cam/scsi/scsi_da.c
Use an ordered command for SCSI/ATA-NCQ commands issued in
response to bios with the BIO_ORDERED flag set.

sys/cam/scsi/scsi_da.c
Use an ordered tag when issuing a synchronize cache command.

Wrap some lines to 80 columns.

sys/cddl/contrib/opensolaris/uts/common/fs/zfs/vdev_geom.c
sys/geom/geom_io.c
Mark bios with the BIO_FLUSH command as BIO_ORDERED.

Sponsored by: Spectra Logic Corporation
MFC after: 1 month

Revert r210225 - turns out I was wrong; the "/*-" is not license-only
thing; it's also used to indicate that the comment should not be automatically
rewrapped.

Explained by: cperciva@

The "/*-" comment marker is supposed to denote copyrights. Remove non-copyright
occurences from sys/sys/ and sys/kern/.

Clarify and reimplement the bioq API so that bioq_disksort() has
the correct behaviour (sorting by distance from the current head position
in the scan direction) and bioq_insert_head() and bioq_insert_tail()
have a well defined (and useful) behaviour, especially when intermixed
with calls to bioq_disksort().

In particular:
- fix a bug in the existing bioq_disksort() that did not use the
current head position correctly;
- redefine semantics of bioq_insert_head() and bioq_insert_tail().
bioq_insert_tail() can now be used as a barrier
between previous and subsequent calls to bioq_disksort().

The code is heavily documented in the source code so please refer
to that for the details.

Much of this code comes from Fabio Checconi. Also thanks to Kirk
for feedback on the (re)definition of bioq_insert_tail().

NOTE: in the current tree there is only a handful of files which
intermix calls to bioq_disksort() with bioq_insert_head() and
bioq_insert_tail(). The ordering of the queue in these situation
was not specified (nor easy to figure out) before, so I doubt any
of that code could be affected by the specification of the API.

Also note that the current implementation is significantly simpler
than the previous one (also used in ata_sort_queue()).
It would be useful to reimplement ata_sort_queue() using
the same code used in bioq_disksort().

MFC after: 1 week

Make bioq_disksort have a ANSI-C definition rather than a K&R definition.

Add a new I/O request - BIO_FLUSH, which basically tells providers below to
flush their caches. For now will mostly be used by disks to flush their
write cache.

Sponsored by: home.pl

Unexpand TAILQ_FIRST(foo) == NULL to TAILQ_EMPTY(foo).

When calling bioq_first() to see if a queue is empty in bioq_disksort(),
don't save the return value as we won't use it.

Noticed by: Coverity Prevent analysis tool
MFC after: 3 days

- Fix insertions of bios which represent data earlier than anything else
in the queue. The insertion sort assumed this had already been taken
care of.

Spotted by: Antoine Brodin
Approved by: re (scottl)

- Dramatically simplify bioqdisksort(). We no longer do ordered bios so
most of the code to deal with them has been dead for sometime. Simplify
the code by doing an insert sort hinted by the current head position.

Met with apathy by: arch@

/* -> /*- for copyright notices, minor format tweaks as necessary

Add bioq_insert_head() function.

OK'd by: phk

Add bioq_takefirst().

If the bioq is empty, NULL is returned. Otherwise the front element
is removed and returned.

This can simplify locking in many drivers from:

lock()
bp = bioq_first(bq);
if (bp == NULL) {
unlock()
return
}
bioq_remove(bp, bq)
unlock
to:
lock()
bp = bioq_takefirst(bq);
unlock()
if (bp == NULL)
return;

Report bio_pblkbo instead of bio_blkno.

Make bioq_disksort() sort on the bio_offset field instead of bio_pblkno.

Made use of 'error' argument, which was unused (by mistake) before.

Submitted by: Pawel Jakub Dawidek <nick@garage.freebsd.pl>

Use __FBSDID().

Don't include <sys/disklabel.h>

Remove BIO_SETATTR from non-GEOM part of kernel as well.

#include <geom/geom_disk.h>

Introduce bioq_flush() function.

retire the "busy" field in bioqueues, it's served it's purpose.

Preparation commit before I start on the bioqueue lockdown:

Collect all the bits of bioqueue handing in subr_disk.c, vfs_bio.c is big
enough as it is and disksort already lives in subr_disk.c.

Including <sys/stdint.h> is (almost?) universally only to be able to use
%j in printfs, so put a newsted include in <sys/systm.h> where the printf
prototype lives and save everybody else the trouble.

Don't pick up a name from the dev_t if it is not there.

NO_GEOM cleanup: remove #ifdef

Remove M_TRYWAIT/M_WAITOK/M_WAIT. Callers should use 0.
Merge M_NOWAIT/M_DONTWAIT into a single flag M_NOWAIT.

Only include <sys/diskslice.h> ifdef NO_GEOM

This checkin reimplements the io-request priority hack in a way
that works in the new threaded kernel. It was commented out of
the disksort routine earlier this year for the reasons given in
kern/subr_disklabel.c (which is where this code used to reside
before it moved to kern/subr_disk.c):

----------------------------
revision 1.65
date: 2002/04/22 06:53:20; author: phk; state: Exp; lines: +5 -0
Comment out Kirks io-request priority hack until we can do this in a
civilized way which doesn't cause grief.

The problem is that it is not generally safe to cast a "struct bio
*" to a "struct buf *". Things like ccd, vinum, ata-raid and GEOM
constructs bio's which are not entrails of a struct buf.

Also, curthread may or may not have anything to do with the I/O request
at hand.

The correct solution can either be to tag struct bio's with a
priority derived from the requesting threads nice and have disksort
act on this field, this wouldn't address the "silly-seek syndrome"
where two equal processes bang the diskheads from one edge to the
other of the disk repeatedly.

Alternatively, and probably better: a sleep should be introduced
either at the time the I/O is requested or at the time it is completed
where we can be sure to sleep in the right thread.

The sleep also needs to be in constant timeunits, 1/hz can be practicaly
any sub-second size, at high HZ the current code practically doesn't
do anything.
----------------------------

As suggested in this comment, it is no longer located in the disk sort
routine, but rather now resides in spec_strategy where the disk operations
are being queued by the thread that is associated with the process that
is really requesting the I/O. At that point, the disk queues are not
visible, so the I/O for positively niced processes is always slowed
down whether or not there is other activity on the disk.

On the issue of scaling HZ, I believe that the current scheme is
better than using a fixed quantum of time. As machines and I/O
subsystems get faster, the resolution on the clock also rises.
So, ten years from now we will be slowing things down for shorter
periods of time, but the proportional effect on the system will
be about the same as it is today. So, I view this as a feature
rather than a drawback. Hence this patch sticks with using HZ.

Sponsored by: DARPA & NAI Labs.
Reviewed by: Poul-Henning Kamp <phk@critter.freebsd.dk>

One #include <sys/sysctl.h> should be enough.

Approved by: mux (mentor)

Separate fiels reported by disk_err() with spaces, so that output doesn't
look cryptic.

MFC after: 1 week

Populate more fields of the disklabel for PC98.

Submitted by: Kawanobe Koh <kawanobe@st.rim.or.jp>

NB: This commit does *NOT* make GEOM the default in FreeBSD
NB: But it will enable it in all kernels not having options "NO_GEOM"

Put the GEOM related options into the intended order.

Add "options NO_GEOM" to all kernel configs apart from NOTES.

In some order of controlled fashion, the NO_GEOM options will be
removed, architecture by architecture in the coming days.

There are currently three known issues which may force people to
need the NO_GEOM option:

boot0cfg/fdisk:
Tries to update the MBR while it is being used to control
slices. GEOM does not allow this as a direct operation.

SCSI floppy drives:
Appearantly the scsi-da driver return "EBUSY" if no media
is inserted. This is wrong, it should return ENXIO.

PC98:
It is unclear if GEOM correctly recognizes all variants of
PC98 disklabels. (Help Wanted! I have neither docs nor HW)

These issues are all being worked.

Sponsored by: DARPA & NAI Labs.

If dsgetlabel() returns a label with a size of zero in diskdumpconf(),
treat it as an invalid partition.

This fixes a bug where ``dumpon <device>'' will configure the dump
device at a random offset on the disk if <device> isn't a valid
partition.

Reviewed by: phk

(This commit touches about 15 disk device drivers in a very consistent
and predictable way, and I apologize if I have gotten it wrong anywhere,
getting prior review on a patch like this is not feasible, considering
the number of people involved and hardware availability etc.)

If struct disklabel is the messenger: kill the messenger.

Inside struct disk we had a struct disklabel which disk drivers used to
communicate certain metrics to the disklayer above (GEOM or the disk
mini-layer). This commit changes this communication to use four
explicit fields instead.

Amongst the benefits is that the fields do not get overwritten by
wrong or bogus on-disk disklabels.

Once that is clear, <sys/disk.h> which is included in the drivers
no longer need to pull <sys/disklabel.h> and <sys/diskslice.h> in,
the few places that needs them, have gotten explicit #includes for
them.

The disklabel inside struct disk is now only for internal use in
the disk mini-layer, so instead of embedding it, we malloc it as
we need it.

This concludes (modulus any mistakes) the series of disklabel related
commits.

I belive it all amounts to a NOP for all the rest of you :-)

Sponsored by: DARPA & NAI Labs.

Make FreeBSD "struct disklabel" agnostic, step 312 of 723:

Rename bioqdisksort() to bioq_disksort().
Keep a #define around to avoid changing all diskdrivers right now.

Move it from subr_disklabel.c to subr_disk.c.
Move prototype from <sys/disklabel.h> to <sys/bio.h>

Sponsored by: DARPA and NAI Labs.

Make FreeBSD "struct disklabel" agnostic, step 311 of 723:

Rename diskerr() to disk_err() for naming consistency.

Drop the by now entirely useless struct disklabel argument.

Add a flag argument for new-line termination.

Fix a couple of printf-format-casts to %j instead of %l.

Correctly print the name of all bio commands.

Move the function from subr_disklabel.c to subr_disk.c,
and from <sys/disklabel.h> to <sys/disk.h>.

Use the new disk_err() throughout, #include <sys/disk.h> as needed.

Bump __FreeBSD_version for the sake of the aac disk drivers #ifdefs.

Remove unused disklabel members of softc for aac, amr and mlx, which seem
to originally have been intended for diskerr() use, but which only rotted
and got Copy&Pasted at least two times to many.

Sponsored by: DARPA & NAI Labs.

Don't use "NULL" when "0" is really meant.

Implement DIOCGFRONTSTUFF ioctl which reports how many bytes from the start
of the device magic stuff might occupy.

Sponsored by: DARPA & NAI Labs.

Rename DIOCGKERNELDUMP to DIOCSKERNELDUMP as it strictly speaking
is a "set" not a "get" operation.

Sponsored by: DARPA & NAI Labs.

Here follows the new kernel dumping infrastructure.

Caveats:

The new savecore program is not complete in the sense that it emulates
enough of the old savecores features to do the job, but implements none
of the options yet.

I would appreciate if a userland hacker could help me out getting savecore
to do what we want it to do from a users point of view, compression,
email-notification, space reservation etc etc. (send me email if
you are interested).

Currently, savecore will scan all devices marked as "swap" or "dump" in
/etc/fstab _or_ any devices specified on the command-line.

All architectures but i386 lack an implementation of dumpsys(), but
looking at the i386 version it should be trivial for anybody familiar
with the platform(s) to provide this function.

Documentation is quite sparse at this time, more to come.

Details:

ATA and SCSI drivers should work as the dump formatting code has been
removed. The IDA, TWE and AAC have not yet been converted.

Dumpon now opens the device and uses ioctl(DIOCGKERNELDUMP) to set
the device as dumpdev. To implement the "off" argument, /dev/null
is used as the device.

Savecore will fail if handed any options since they are not (yet)
implemented. All devices marked "dump" or "swap" in /etc/fstab
will be scanned and dumps found will be saved to diskfiles
named from the MD5 hash of the header record. The header record
is dumped in readable format in the .info file. The kernel
is not saved. Only complete dumps will be saved.

All maintainer rights for this code are disclaimed: feel free to
improve and extend.

Sponsored by: DARPA, NAI Labs

Make the disk_clone() routine more robust for abuse.
Sneak in a trivial bit of the GEOM stuff while we're here anyway.

Fix a warning.

Don't call cdevsw_add().

Rename the top 7 bits if disk minors to spare bits, rather than type bits.

Don't choke on old sd%d.ctl devices.

Tripped over by: Jos Backus <josb@cncdsl.com>

Turn the symlinks around, instead of ad0s1 -> ad0s1c, make it ad0s1c -> ad0s1.

Requested by: peter

Fix a problem in the disk related hack where device nodes for a physically
non-existent disk in a legacy /dev on a DEVFS system would panic the system
if stat(2)'ed.

Do not whine about anonymous device nodes not having a si_devsw, they're
not supposed to.

Nudge the axe a bit closer to cdevsw[]:

Make it a panic to repeat make_dev() or destroy_dev(), this check
should maybe be neutered when -current goes -stable.

Whine if devsw() is called on anon dev_t's in a devfs system.

Make a hack to avoid our lazy-eval disk code triggering the above whine.

Fix the multiple make_dev() in disk code by making ${disk}${unit}s${slice}
an alias/symlink to ${disk}${unit}s${slice}c

disk_clone() was a bit too eager to please: "md0s1ec" is not a valid
device.

Noticed by: Chad David <davidc@acns.ab.ca>

KSE Milestone 2
Note ALL MODULES MUST BE RECOMPILED
make the kernel aware that there are smaller units of scheduling than the
process. (but only allow one thread per process at this time).
This is functionally equivalent to teh previousl -current except
that there is a thread associated with each process.

Sorry john! (your next MFC will be a doosie!)

Reviewed by: peter@freebsd.org, dillon@freebsd.org

X-MFC after: ha ha ha ha

Don't dump on the label sector or below. This avoids clobbering the
label if the dump device overflaps the label (which is a slight
misconfiguration). Dump routines don't use dscheck(), so the normal
write protection of the label doesn't help.

Reduced some nearby overflow bugs. In disk_dumpcheck(), there was
(fatal but fail-safe) overflow on i386's with 4GB of memory, at least
if Maxmem was the top page (can this happen?). The fix assumes that
the sector size divides PAGE_SIZE (dump routines already assume this).
In setdumpdev(), the corresponding overflow occurred with only about
2GB of memory on all machines with 32-bit ints. This allowed setdumpdev()
to succeed when it shouldn't have, but then disk_dumpcheck() failed
safe later. Except in old versions of FreeBSD like RELENG_3 where
there is no disk_dumpcheck().

PR: 28164 (label clobbering part)
MFC after: 1 week

Remove the hack-around for the slice/label code, it didn't
cover the hole.

The disklabel/slice code is more twisted than I thought. Revert to
calling the cdevsw_add() unconditionally.

Create a general facility for making dev_t's depend on another
dev_t. The dev_depends(dev_t, dev_t) function is for tying them
to each other.

When destroy_dev() is called on a dev_t, all dev_t's depending
on it will also be destroyed (depth first order).

Rewrite the make_dev_alias() to use this dependency facility.

kern/subr_disk.c:
Make the disk mini-layer use dependencies to make sure all
relevant dev_t's are removed when the disk disappears.

Make the disk mini-layer precreate some magic sub devices
which the disk/slice/label code expects to be there.

kern/subr_disklabel.c:
Remove some now unneeded variables.

kern/subr_diskmbr.c:
Remove some ancient, commented out code.

kern/subr_diskslice.c:
Minor cleanup. Use name from dev_t instead of dsname()

Don't take the detour around devsw() to find out if the proto-cdevsw
is already initialized.

Always initialize bio_resid from bio_bcount in the disk mini-layer so
that the drivers don't have to do it umpteen times.

Make the disk mini-layer check for and handle zero-length transfers
instead of the underlying drivers.

Actually biofinish(struct bio *, struct devstat *, int error) is more general
than the bioerror().

Most of this patch is generated by scripts.

Dont call device close and ioctl functions if device has disappeared.

Reviewed by: phk

Don't clone impossible unit numbers for disks.

Staticize some malloc M_ instances.

Avoid the modules madness I inadvertently introduced by making the
cloning infrastructure standard in kern_conf. Modules are now
the same with or without devfs support.

If you need to detect if devfs is present, in modules or elsewhere,
check the integer variable "devfs_present".

This happily removes an ugly hack from kern/vfs_conf.c.

This forces a rename of the eventhandler and the standard clone
helper function.

Include <sys/eventhandler.h> in <sys/conf.h>: it's a helper #include
like <sys/queue.h>

Remove all #includes of opt_devfs.h they no longer matter.

Remove all traces of Julians DEVFS (incl from kern/subr_diskslice.c)

Remove old DEVFS support fields from dev_t.

Make uid, gid & mode members of dev_t and set them in make_dev().

Use correct uid, gid & mode in make_dev in disk minilayer.

Add support for registering alias names for a dev_t using the
new function make_dev_alias(). These will show up as symlinks
in DEVFS.

Use makedev() rather than make_dev() for MFSs magic devices to prevent
DEVFS from noticing this abuse.

Add a field for DEVFS inode number in dev_t.

Add new DEVFS in fs/devfs.

Add devfs cloning to:
disk minilayer (ie: ad(4), sd(4), cd(4) etc etc)
md(4), tun(4), bpf(4), fd(4)

If DEVFS add -d flag to /sbin/inits args to make it mount devfs.

Add commented out DEVFS to GENERIC

End two weeks of on and off debugging. Fix the crash on the Nth
insertion of a CF card, for random values of N > 1. With these fixes,
I've been able to do 100 insert/remove of the cards w/o a crash with
lots of system activity going on that in the past would help trigger
the crash.

The problem:

FreeBSD creates dev_t's on the fly as they are needed and never
destroys them. These dev_t's point to a struct disk that is used for
housekeeping on the disk. When a device goes away, the struct disk
pointer becomes a dangling pointer. Sometimes when the device comes
back, the pointer will point to the new struct disk (in which case the
insertion will work). Other times it won't (especially if any length
of time has passed, since it is dependent on memory returned from
malloc).

The Fix:

There is one of these dev_t's that is always correct. The
device for the WHOLE_DISK_SLICE is always right. It gets set at
create_disk() time. So, the fix is to spend a little CPU time and
lookup the WHOLE_DISK_SLICE dev_t and use the si_disk from that in
preference to the one that's in the device asking to do the I/O. In
addition, we change the test of si_disk == NULL meaning that the dev
needed to inherit properties from the pdev to dev->si_disk !=
pdev->si_disk. This test is a little stronger than the previous test,
but can sometimes be fooled into not inheriting. However, the results
of this fooling are that the old values will be used, which will
generally always be the same as before. si_drv[12] are the only
values that are copied that might pose a problem. They tend to change
as the si_disk field would change, so it is a hole, but it is a small
hole.

One could correctly argue that one should replace much of this code
with something much much better. I would be on the pro side of that
argument.

Reviewed by: phk (who also ported the original patch to current)
Sponsored by: Timing Solutions

Previous commit changing SYSCTL_HANDLER_ARGS violated KNF.

Pointed out by: bde

Style police catches up with rev 1.26 of src/sys/sys/sysctl.h:

Sanitize SYSCTL_HANDLER_ARGS so that simplistic tools can grog our
sources:

-sysctl_vm_zone SYSCTL_HANDLER_ARGS
+sysctl_vm_zone (SYSCTL_HANDLER_ARGS)

Add 'kern.disks', a sysctl which returns the list of disks from
disk_enumerate(), space delimited. This allows non-root users to get a
list of disks and will simplify libdisk's Disk_Names().

Reviewed by: phk

Add disk_enumerate() for finding names of disks. Vinum and libh will
need this RSN.

Remove a pointless warning in the root device locating code.

Remove the "wd" compatibility name from the "ad" driver.

WARNING: If you have not updated to use /dev/wd* in your /etc/fstab
and modern bootblocks, it would be a very good idea to do so BEFORE
you upgrade your kernel.

Separate the struct bio related stuff out of <sys/buf.h> into
<sys/bio.h>.

<sys/bio.h> is now a prerequisite for <sys/buf.h> but it shall
not be made a nested include according to bdes teachings on the
subject of nested includes.

Diskdrivers and similar stuff below specfs::strategy() should no
longer need to include <sys/buf.> unless they need caching of data.

Still a few bogus uses of struct buf to track down.

Repocopy by: peter

Clone the {b|bio}_offset field, and make sure it is always initialized
in struct bio. Eventually, bio_offset will probably obsolete the
bio_blkno and bio_pblkno fields.

Remove the special hack in atapi-cd.c to determine of bio_offset was valid.

Complete the bio/buf divorce for all code below devfs::strategy

Exceptions:
Vinum untouched. This means that it cannot be compiled.
Greg Lehey is on the case.

CCD not converted yet, casts to struct buf (still safe)

atapi-cd casts to struct buf to examine B_PHYS

Move B_ERROR flag to b_ioflags and call it BIO_ERROR.

(Much of this done by script)

Move B_ORDERED flag to b_ioflags and call it BIO_ORDERED.

Move b_pblkno and b_iodone_chain to struct bio while we transition, they
will be obsoleted once bio structs chain/stack.

Add bio_queue field for struct bio aware disksort.

Address a lot of stylistic issues brought up by bde.

Fixed a null pointer panic for dumpon(8) on a nonexistent device whose
driver uses the new disk layer.

Reviewed by: phk
Approved by: jkh

Update the ata driver to take more advantage of newbus, this
was needed to make attach/detach of devices work, which is
needed for the PCCARD support.
(PCCARD support is still not working though, more to come on that)

Support the CMD646 chip which is used on many alphas, sadly only
in WDMA2 mode, as the silicon is broken beyond belief for UDMA modes.

Lots of cosmetic fixes here and there.

Sorry for the size of this megapatchfromhell but it was not
possible otherwise...

newbus patches based on work from: dfr (Doug Rabson)

rename disk_delete() to disk_destroy().

Also handle zero return from dscheck().

PR: 15956

Don't ignore return value from tsleep().

Spotted by: charnier

Conditionalise unwanted chattyness.

Put a lock on the disk structure while we open to avoid races.

PR: 14486

Trim unused options (or #ifdef for undoc options).

Submitted by: phk

be more consistent about passing the whole/raw dev_t to the driver

In some drivers we use two devices to be able to boot.
So if si_iosize_max is allready set, dont mess with it..

Also just log the problem with maxphys not being set once.

designed by: phk
tested by: sos

Fix a problem relating to si_iosize_max which broke scsi devices.

Make all slices/partitions correctly inherit si_* fields.

Lightly tested by: msmith

Fix disk_close once more, and better this time.

Spotted by: bde

Test the slices for openness before we close them; doing it the other way
around meant that the higher level close routine never gets called.
(phk is on the road; this is a quick fix to get things working and may need
more polish)

Register the right cdevsw on the master device.

Detected by: sos

Bite the bullet and allocate the devsw entry at compile time.

Use a different tactic when creating the devsw so that disk_create()
doesn't need to malloc.

Changes to centralise the default blocksize behaviour.
More likely to follow.

Submitted by: phk@freebsd.org

Improve the micro "disk" layer after gaining more experience with it.

Add micro "disk" layer which should enable us to pull all the slice/label
stuff out of the device drivers.