geom/zero: Add support for unmapped I/OHEADmain

This patch adds support for unmapped I/O to gzero(4).

Let's consider the following script to illustrate the change in
gzero(4)'s behavior:

```
dd="dd if=/dev/gzero of=/dev/null bs=512 count=100000"
dtrace -q -c "$dd" -n '
    fbt::pmap_qenter:entry,
    fbt::uiomove_fromphys:entry,
    fbt::memset:entry
    /execname == "dd"/
    {
	@[probefunc] = count();
    }
'
```

Let's run that script 4 times:

```
==> 1: unmapped I/O not supported (fallback to mapped I/O), kern.geom.zero.clear=1
51200000 bytes transferred in 1.795809 secs (28510829 bytes/sec)
  pmap_qenter                                                  100000
  memset                                                       400011

==> 2: unmapped I/O not supported (fallback to mapped I/O), kern.geom.zero.clear=0
51200000 bytes transferred in 0.701079 secs (73030337 bytes/sec)
  memset                                                       300011

==> 3: unmapped I/O supported, kern.geom.zero.clear=1
51200000 bytes transferred in 0.771680 secs (66348750 bytes/sec)
  uiomove_fromphys                                             100000
  memset                                                       300011

==> 4: unmapped I/O supported, kern.geom.zero.clear=0
51200000 bytes transferred in 0.621303 secs (82407407 bytes/sec)
  memset                                                       300011
```

If kern.geom.zero.clear=0, then nothing really changes as no copying takes
place. Otherwise, we see by adding unmapped I/O support we avoid calls to
pmap_qenter(), which was called by GEOM to turn unmapped I/O requests into
mapped ones before passing them for processing to gzero(4).

Reviewed by:	bnovkov, markj
Approved by:	bnovkov (mentor), markj (mentor)
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D52998

0 files changed, 0 insertions, 0 deletions