This patch introduces bio-based IO path for virtio-blk.

Compared to request-based IO path, bio-based IO path uses driver
provided ->make_request_fn() method to bypasses the IO scheduler. It
handles the bio to device directly without allocating a request in block
layer. This reduces the IO path in guest kernel to achieve high IOPS
and lower latency. The downside is that guest can not use the IO
scheduler to merge and sort requests. However, this is not a big problem
if the backend disk in host side uses faster disk device.

When the bio-based IO path is not enabled, virtio-blk still uses the
original request-based IO path, no performance difference is observed.

Using a slow device e.g. normal SATA disk, the bio-based IO path for
sequential read and write are slower than req-based IO path due to lack
of merge in guest kernel. So we make the bio-based path optional.

Performance evaluation:
-----------------------------
1) Fio test is performed in a 8 vcpu guest with ramdisk based guest using
kvm tool.

Short version:
 With bio-based IO path, sequential read/write, random read/write
 IOPS boost         : 28%, 24%, 21%, 16%
 Latency improvement: 32%, 17%, 21%, 16%

Long version:
 With bio-based IO path:
  seq-read  : io=2048.0MB, bw=116996KB/s, iops=233991 , runt= 17925msec
  seq-write : io=2048.0MB, bw=100829KB/s, iops=201658 , runt= 20799msec
  rand-read : io=3095.7MB, bw=112134KB/s, iops=224268 , runt= 28269msec
  rand-write: io=3095.7MB, bw=96198KB/s,  iops=192396 , runt= 32952msec
    clat (usec): min=0 , max=2631.6K, avg=58716.99, stdev=191377.30
    clat (usec): min=0 , max=1753.2K, avg=66423.25, stdev=81774.35
    clat (usec): min=0 , max=2915.5K, avg=61685.70, stdev=120598.39
    clat (usec): min=0 , max=1933.4K, avg=76935.12, stdev=96603.45
  cpu : usr=74.08%, sys=703.84%, ctx=29661403, majf=21354, minf=22460954
  cpu : usr=70.92%, sys=702.81%, ctx=77219828, majf=13980, minf=27713137
  cpu : usr=72.23%, sys=695.37%, ctx=88081059, majf=18475, minf=28177648
  cpu : usr=69.69%, sys=654.13%, ctx=145476035, majf=15867, minf=26176375
 With request-based IO path:
  seq-read  : io=2048.0MB, bw=91074KB/s, iops=182147 , runt= 23027msec
  seq-write : io=2048.0MB, bw=80725KB/s, iops=161449 , runt= 25979msec
  rand-read : io=3095.7MB, bw=92106KB/s, iops=184211 , runt= 34416msec
  rand-write: io=3095.7MB, bw=82815KB/s, iops=165630 , runt= 38277msec
    clat (usec): min=0 , max=1932.4K, avg=77824.17, stdev=170339.49
    clat (usec): min=0 , max=2510.2K, avg=78023.96, stdev=146949.15
    clat (usec): min=0 , max=3037.2K, avg=74746.53, stdev=128498.27
    clat (usec): min=0 , max=1363.4K, avg=89830.75, stdev=114279.68
  cpu : usr=53.28%, sys=724.19%, ctx=37988895, majf=17531, minf=23577622
  cpu : usr=49.03%, sys=633.20%, ctx=205935380, majf=18197, minf=27288959
  cpu : usr=55.78%, sys=722.40%, ctx=101525058, majf=19273, minf=28067082
  cpu : usr=56.55%, sys=690.83%, ctx=228205022, majf=18039, minf=26551985

2) Fio test is performed in a 8 vcpu guest with Fusion-IO based guest using
kvm tool.

Short version:
 With bio-based IO path, sequential read/write, random read/write
 IOPS boost         : 11%, 11%, 13%, 10%
 Latency improvement: 10%, 10%, 12%, 10%
Long Version:
 With bio-based IO path:
  read : io=2048.0MB, bw=58920KB/s, iops=117840 , runt= 35593msec
  write: io=2048.0MB, bw=64308KB/s, iops=128616 , runt= 32611msec
  read : io=3095.7MB, bw=59633KB/s, iops=119266 , runt= 53157msec
  write: io=3095.7MB, bw=62993KB/s, iops=125985 , runt= 50322msec
    clat (usec): min=0 , max=1284.3K, avg=128109.01, stdev=71513.29
    clat (usec): min=94 , max=962339 , avg=116832.95, stdev=65836.80
    clat (usec): min=0 , max=1846.6K, avg=128509.99, stdev=89575.07
    clat (usec): min=0 , max=2256.4K, avg=121361.84, stdev=82747.25
  cpu : usr=56.79%, sys=421.70%, ctx=147335118, majf=21080, minf=19852517
  cpu : usr=61.81%, sys=455.53%, ctx=143269950, majf=16027, minf=24800604
  cpu : usr=63.10%, sys=455.38%, ctx=178373538, majf=16958, minf=24822612
  cpu : usr=62.04%, sys=453.58%, ctx=226902362, majf=16089, minf=23278105
 With request-based IO path:
  read : io=2048.0MB, bw=52896KB/s, iops=105791 , runt= 39647msec
  write: io=2048.0MB, bw=57856KB/s, iops=115711 , runt= 36248msec
  read : io=3095.7MB, bw=52387KB/s, iops=104773 , runt= 60510msec
  write: io=3095.7MB, bw=57310KB/s, iops=114619 , runt= 55312msec
    clat (usec): min=0 , max=1532.6K, avg=142085.62, stdev=109196.84
    clat (usec): min=0 , max=1487.4K, avg=129110.71, stdev=114973.64
    clat (usec): min=0 , max=1388.6K, avg=145049.22, stdev=107232.55
    clat (usec): min=0 , max=1465.9K, avg=133585.67, stdev=110322.95
  cpu : usr=44.08%, sys=590.71%, ctx=451812322, majf=14841, minf=17648641
  cpu : usr=48.73%, sys=610.78%, ctx=418953997, majf=22164, minf=26850689
  cpu : usr=45.58%, sys=581.16%, ctx=714079216, majf=21497, minf=22558223
  cpu : usr=48.40%, sys=599.65%, ctx=656089423, majf=16393, minf=23824409

3) Fio test is performed in a 8 vcpu guest with normal SATA based guest
using kvm tool.

Short version:
 With bio-based IO path, sequential read/write, random read/write
 IOPS boost         : -10%, -10%, 4.4%, 0.5%
 Latency improvement: -12%, -15%, 2.5%, 0.8%
Long Version:
 With bio-based IO path:
  read : io=124812KB, bw=36537KB/s, iops=9060 , runt=  3416msec
  write: io=169180KB, bw=24406KB/s, iops=6065 , runt=  6932msec
  read : io=256200KB, bw=2089.3KB/s, iops=520 , runt=122630msec
  write: io=257988KB, bw=1545.7KB/s, iops=384 , runt=166910msec
    clat (msec): min=1 , max=1527 , avg=28.06, stdev=89.54
    clat (msec): min=2 , max=344 , avg=41.12, stdev=38.70
    clat (msec): min=8 , max=1984 , avg=490.63, stdev=207.28
    clat (msec): min=33 , max=4131 , avg=659.19, stdev=304.71
  cpu          : usr=4.85%, sys=17.15%, ctx=31593, majf=0, minf=7
  cpu          : usr=3.04%, sys=11.45%, ctx=39377, majf=0, minf=0
  cpu          : usr=0.47%, sys=1.59%, ctx=262986, majf=0, minf=16
  cpu          : usr=0.47%, sys=1.46%, ctx=337410, majf=0, minf=0

 With request-based IO path:
  read : io=150120KB, bw=40420KB/s, iops=10037 , runt=  3714msec
  write: io=194932KB, bw=27029KB/s, iops=6722 , runt=  7212msec
  read : io=257136KB, bw=2001.1KB/s, iops=498 , runt=128443msec
  write: io=258276KB, bw=1537.2KB/s, iops=382 , runt=168028msec
    clat (msec): min=1 , max=1542 , avg=24.84, stdev=32.45
    clat (msec): min=3 , max=628 , avg=35.62, stdev=39.71
    clat (msec): min=8 , max=2540 , avg=503.28, stdev=236.97
    clat (msec): min=41 , max=4398 , avg=653.88, stdev=302.61
  cpu          : usr=3.91%, sys=15.75%, ctx=26968, majf=0, minf=23
  cpu          : usr=2.50%, sys=10.56%, ctx=19090, majf=0, minf=0
  cpu          : usr=0.16%, sys=0.43%, ctx=20159, majf=0, minf=16
  cpu          : usr=0.18%, sys=0.53%, ctx=81364, majf=0, minf=0

How to use:
-----------------------------
Add 'virtio_blk.use_bio=1' to kernel cmdline or 'modprobe virtio_blk
use_bio=1' to enable ->make_request_fn() based I/O path.

Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Tejun Heo <tj@kernel.org>
Cc: Shaohua Li <shli@kernel.org>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: kvm@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: virtualization@lists.linux-foundation.org
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Minchan Kim <minchan.kim@gmail.com>
Signed-off-by: Asias He <asias@redhat.com>
Acked-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>