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+.\"	$NetBSD: viomb.4,v 1.4 2020/08/24 19:27:36 ryoon Exp $
+.\"
+.\" Copyright (C) 2011 Minoura Makoto.
+.\" All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
+.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+.\" BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+.\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+.\" AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+.\" OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+.\" SUCH DAMAGE.
+.\"
+.Dd August 25, 2020
+.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 ,
+.Xr x86/balloon 4
+.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.