acpivmgenid(4) - NetBSD Manual Pages

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ACPIVMGENID(4)          NetBSD Kernel Interfaces Manual         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 mea-
     sure of zero entropy) so that the two virtual machines will produce inde-
     pendent 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,
     http://go.microsoft.com/fwlink/?LinkId=260709, Microsoft, 2018-08-01.

     Virtual Machine Generation ID Device,
     https://www.qemu.org/docs/master/specs/vmgenid.html, The QEMU Project
     Developers.


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.)

NetBSD 10.1                     August 26, 2024                    NetBSD 10.1