.\"	$NetBSD: viomb.4,v 1.3 2012/03/08 22:52:22 wiz Exp $
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.\" Copyright (C) 2011 Minoura Makoto.
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.Dd November 26, 2011
.Dt VIOMB 4
.Os
.Sh NAME
.Nm viomb
.Nd VirtIO memory ballooning driver
.Sh SYNOPSIS
.Cd "virtio* at pci? dev ? function ?"
.Cd "viomb*  at virtio?"
.Sh DESCRIPTION
.Xr virtio 4
defines an interface for efficient, standard, and extensible I/O between
the hypervisor and the virtual machine.
The
.Nm
driver supports the virtio-compliant memory ballooning device.
.Pp
Memory ballooning works as follows:
.Pp
.Bl -enum -compact
.It
The host operator requests a guest to return some amount of memory to the host
(via e.g. Qemu monitor balloon command).
.It
The hypervisor sends the request via VirtIO memory ballooning device.
.It
The guest
.Nm
driver requests allocation of that amount of physical memory from the
.Nx
memory management system.
.It
The
.Nm
device tells the hypervisor the guest physical memory address of the
allocated memory via VirtIO memory ballooning device.
.El
.Pp
The sysctl node
.Li hw.viomb.npages
shows the requested number of memory pages to return to the hypervisor, while
.Li hw.viomb.actual
shows the actual number of memory pages that are already returned to the hypervisor.
.Sh SEE ALSO
.Xr virtio 4 ,
.Xr sysctl 8
.Pp
.Rs
.%A Rusty Russell, IBM Corporation
.%T Virtio PCI Card Specification
.%U http://ozlabs.org/~rusty/virtio-spec/
.Re
.Sh HISTORY
The
.Nm
device driver appeared in
.Nx 6.0 .
.Sh BUGS
The userland interface should be same as the Xen ballooning device.