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<table class="head">
<tr>
<td class="head-ltitle">VMGENC(4)</td>
<td class="head-vol">Device Drivers Manual</td>
<td class="head-rtitle">VMGENC(4)</td>
</tr>
</table>
<div class="manual-text">
<section class="Sh">
<h1 class="Sh" id="NAME"><a class="permalink" href="#NAME">NAME</a></h1>
<p class="Pp"><code class="Nm">vmgenc</code> — <span class="Nd">ACPI
virtual machine generation ID counter</span></p>
</section>
<section class="Sh">
<h1 class="Sh" id="SYNOPSIS"><a class="permalink" href="#SYNOPSIS">SYNOPSIS</a></h1>
<p class="Pp"><code class="Cd">device vmgenc</code></p>
<p class="Pp">In <a class="Xr">loader.conf(5)</a>:
<br/>
<code class="Cd">vmgenc_load="YES"</code></p>
</section>
<section class="Sh">
<h1 class="Sh" id="DESCRIPTION"><a class="permalink" href="#DESCRIPTION">DESCRIPTION</a></h1>
<p class="Pp">The <code class="Nm">vmgenc</code> driver provides support for the
Virtual Machine Generation ID, a 128-bit unique identifier exposed by the
hypervisor via ACPI. The hypervisor changes this identifier whenever the
virtual machine is cloned, restored from a snapshot, or otherwise
duplicated.</p>
<p class="Pp">When a generation ID change is detected, the
<code class="Nm">vmgenc</code> driver feeds the new identifier into the
kernel entropy pool via <a class="Xr">random(4)</a>, ensuring that
duplicated virtual machines do not share cryptographic state. The driver
also sends a <a class="Xr">devctl(4)</a> event and an internal kernel
notification so that other subsystems can respond to the duplication.</p>
<p class="Pp">The Virtual Machine Generation ID specification is supported by
QEMU, VMware ESXi, Microsoft Hyper-V, and Xen.</p>
</section>
<section class="Sh">
<h1 class="Sh" id="SYSCTL_VARIABLES"><a class="permalink" href="#SYSCTL_VARIABLES">SYSCTL
VARIABLES</a></h1>
<p class="Pp">The following variable is available:</p>
<dl class="Bl-tag">
<dt id="dev.vmgenc._d.guid"><var class="Va">dev.vmgenc.%d.guid</var></dt>
<dd>The current cached VM generation counter as a 128-bit value. This value is
updated each time the hypervisor signals a generation change.</dd>
</dl>
</section>
<section class="Sh">
<h1 class="Sh" id="SEE_ALSO"><a class="permalink" href="#SEE_ALSO">SEE
ALSO</a></h1>
<p class="Pp"><a class="Xr">acpi(4)</a>, <a class="Xr">random(4)</a></p>
</section>
<section class="Sh">
<h1 class="Sh" id="HISTORY"><a class="permalink" href="#HISTORY">HISTORY</a></h1>
<p class="Pp">The <code class="Nm">vmgenc</code> driver first appeared in
<span class="Ux">FreeBSD 13.0</span>.</p>
</section>
<section class="Sh">
<h1 class="Sh" id="AUTHORS"><a class="permalink" href="#AUTHORS">AUTHORS</a></h1>
<p class="Pp">The <code class="Nm">vmgenc</code> driver was written by
<span class="An">Conrad Meyer</span>
<<a class="Mt" href="mailto:cem@FreeBSD.org">cem@FreeBSD.org</a>>.</p>
<p class="Pp">This manual page was written by
<br/>
<span class="An">Christos Longros</span>
<<a class="Mt" href="mailto:chris.longros@gmail.com">chris.longros@gmail.com</a>>.</p>
</section>
</div>
<table class="foot">
<tr>
<td class="foot-date">March 21, 2026</td>
<td class="foot-os">FreeBSD 15.0</td>
</tr>
</table>
|
