ACPIVMGENID(4)

NAME
acpivmgenid - ACPI Virtual Machine Generation ID

SYNOPSIS
acpivmgenid* at acpi?

DESCRIPTION
acpivmgenid provides a generation ID for virtual machines.

When starting two otherwise identical virtual machines, whether from the same clean image or by cloning snapshots or any other mechanism, the VM host may choose a different generation ID. Although this generation ID is not secret, it is incorporated into the entropy(7) pool (with a measure of zero entropy) so that the two virtual machines will produce independent random output.

If a live VM is cloned, the VM host may change the generation ID in one or both of the clones and notify them through the acpivmgenid device. When this happens, NetBSD will reseed system random number generators, so that output of /dev/urandom and getentropy(3) will be independent in the two clones, and the sysctl(7) variable kern.entropy.epoch will advance to notify applications that they should reseed random number generators from the system entropy pool.

SYSCTLS
The following sysctl(7) nodes are available:

hw.acpivmgenidN.id
    The current 16-byte VM generation ID.

hw.acpivmgenidN.paddr
    The physical address of the VM generation ID provided by the host.

SEE ALSO
arc4random(3), getentropy(3), rnd(4), entropy(7) Virtual Machine Generation ID, Microsoft, 2018-08-01, http://go.microsoft.com/fwlink/?LinkId=260709. Virtual Machine Generation ID Device, The QEMU Project Developers, https://www.qemu.org/docs/master/specs/vmgenid.html.

HISTORY
The acpivmgenid driver first appeared in NetBSD 10.1.

BUGS
Currently there is no cheaper way to detect VM generation ID changes than to query sysctl. (Applications deciding whether to reseed random number generators should generally query kern.entropy.epoch, not hw.acpivmgenidN.id.)