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 <div class="section" id="subchannel-passthrough-via-vfio-ccw">
<h1>Subchannel passthrough via vfio-ccw<a class="headerlink" href="#subchannel-passthrough-via-vfio-ccw" title="Permalink to this headline">¶</a></h1>
vfio-ccw (based upon the mediated vfio device infrastructure) allows to
make certain I/O subchannels and their devices available to a guest. The
host will not interact with those subchannels/devices any more.
Note that while vfio-ccw should work with most non-QDIO devices, only ECKD
DASDs have really been tested.
<div class="section" id="example-configuration">
<h2>Example configuration<a class="headerlink" href="#example-configuration" title="Permalink to this headline">¶</a></h2>
<div class="section" id="step-1-configure-the-host-device">
<h3>Step 1: configure the host device<a class="headerlink" href="#step-1-configure-the-host-device" title="Permalink to this headline">¶</a></h3>
As every mdev is identified by a uuid, the first step is to obtain one:
<div class="highlight-default notranslate"><div class="highlight"><pre>[root@host ~]# uuidgen
7e270a25-e163-4922-af60-757fc8ed48c6
</pre></div>
</div>
Note: it is recommended to use the <code class="docutils literal notranslate">mdevctl</code> tool for actually configuring
the host device.
To define the same device as configured below to be started
automatically, use
<div class="highlight-default notranslate"><div class="highlight"><pre>[root@host ~]# driverctl -b css set-override 0.0.0313 vfio_ccw
[root@host ~]# mdevctl define -u 7e270a25-e163-4922-af60-757fc8ed48c6 \
 -p 0.0.0313 -t vfio_ccw-io -a
</pre></div>
</div>
If using <code class="docutils literal notranslate">mdevctl</code> is not possible or wanted, follow the manual procedure
below.
<ul>
<li>Locate the subchannel for the device (in this example, <code class="docutils literal notranslate">0.0.2b09</code>):
<div class="highlight-default notranslate"><div class="highlight"><pre>[root@host ~]# lscss | grep 0.0.2b09 | awk &#39;{print $2}&#39;
0.0.0313
</pre></div>
</div>
</li>
<li>Unbind the subchannel (in this example, <code class="docutils literal notranslate">0.0.0313</code>) from the standard
I/O subchannel driver and bind it to the vfio-ccw driver:
<div class="highlight-default notranslate"><div class="highlight"><pre>[root@host ~]# echo 0.0.0313 &gt; /sys/bus/css/devices/0.0.0313/driver/unbind
[root@host ~]# echo 0.0.0313 &gt; /sys/bus/css/drivers/vfio_ccw/bind
</pre></div>
</div>
</li>
<li>Create the mediated device (identified by the uuid):
<div class="highlight-default notranslate"><div class="highlight"><pre>[root@host ~]# echo &quot;7e270a25-e163-4922-af60-757fc8ed48c6&quot; &gt; \
/sys/bus/css/devices/0.0.0313/mdev_supported_types/vfio_ccw-io/create
</pre></div>
</div>
</li>
</ul>
</div>
<div class="section" id="step-2-configure-qemu">
<h3>Step 2: configure QEMU<a class="headerlink" href="#step-2-configure-qemu" title="Permalink to this headline">¶</a></h3>
<ul>
<li>Reference the created mediated device and (optionally) pick a device id to
be presented in the guest (here, <code class="docutils literal notranslate">fe.0.1234</code>, which will end up visible
in the guest as <code class="docutils literal notranslate">0.0.1234</code>:
<div class="highlight-default notranslate"><div class="highlight"><pre>-device vfio-ccw,devno=fe.0.1234,sysfsdev=\
/sys/bus/mdev/devices/7e270a25-e163-4922-af60-757fc8ed48c6
</pre></div>
</div>
</li>
<li>Start the guest. The device (here, <code class="docutils literal notranslate">0.0.1234</code>) should now be usable:
<div class="highlight-default notranslate"><div class="highlight"><pre>[root@guest ~]# lscss -d 0.0.1234
Device Subchan. DevType CU Type Use PIM PAM POM CHPID
----------------------------------------------------------------------
0.0.1234 0.0.0007 3390/0e 3990/e9 f0 f0 ff 1a2a3a0a 00000000
[root@guest ~]# chccwdev -e 0.0.1234
Setting device 0.0.1234 online
Done
[root@guest ~]# dmesg -t
(...)
dasd-eckd 0.0.1234: A channel path to the device has become operational
dasd-eckd 0.0.1234: New DASD 3390/0E (CU 3990/01) with 10017 cylinders, 15 heads, 224 sectors
dasd-eckd 0.0.1234: DASD with 4 KB/block, 7212240 KB total size, 48 KB/track, compatible disk layout
dasda:VOL1/ 0X2B09: dasda1
</pre></div>
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