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.Dd
.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
The VirtIO devices are found in Linux KVM, Qemu, Oracle VM VirtualBox and
other virtual machine guest environment.  The
.Nm
driver supports "Virtio PCI Card Specification"-compliant memory ballooning
interface.
.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 hw.viomb.npages shows the requested number of memory pages
to return to the hypervisor, while hw.viomb.actual shows the actual
number of memory pages that is 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
.%O http://ozlabs.org/~rusty/virtio-spec/
.Re
.Sh HISTORY
The
.Nm
device driver appeared in
.Nx 6.0 .
.Sh BUG
The userland interface should be same as the Xen ballooning device.