\section{Shared Memory Regions}\label{sec:Basic Facilities of a Virtio Device / Shared Memory Regions}

Shared memory regions are an additional facility
available to devices that need a region of memory that's
continuously shared between the device and the driver, rather
than passed between them in the way virtqueue elements are.

Example uses include shared caches and version pools for versioned
data structures.

The memory region is allocated by the device and presented to the
driver.  Where the device is implemented in software on a host,
this arrangement allows the memory region to be allocated by
a library on the host, which the device may not have full control
over.

A device may have multiple shared memory regions associated with
it.  Each region has a \field{shmid} to identify it, the meaning
of which is device-specific.

Enumeration and location of shared memory regions is performed
in a transport-specific way.

Memory consistency rules vary depending on the region and the
device and they will be specified as required by each device.

\subsection{Addressing within regions}\label{sec:Basic Facilities of a Virtio Device / Shared Memory Regions / Addressing within regions }

References into shared memory regions are represented as offsets from
the beginning of the region instead of absolute memory addresses.
Offsets are used both for references between structures stored
within shared memory and for requests placed in virtqueues that
refer to shared memory.
The \field{shmid} may be explicit or may be inferred from the
context of the reference.

\devicenormative{\subsection}{Shared Memory Regions}{Basic Facilities of a Virtio
Device / Shared Memory Regions}
Shared memory regions MUST NOT expose shared memory regions which
are used to control the operation of the device, nor to stream
data.