Guests in a hypervisor system can share memory regions through which they can pass
data to each other or to the hypervisor host.
In a QNX hypervisor system, client applications running in guests can create and
manage shared memory, and use shared memory regions to exchange data. Note, however,
that shared memory regions are ultimately created and controlled by the hypervisor;
not by the guest. Host applications may also create shared memory regions or attach
to them if permission allows.
The hypervisor-shmem-examples-*.tgz archive available with QNX
hypervisors includes source code for sample memory-sharing programs:
ghstest.c for a QNX guest and
hhstest.c for the hypervisor host.
To write hypervisor host modules that can share data with guests, you need to use the
Virtualization API (libhyp.a). This is described in the
Virtualization API Reference that's not included with the
QNX hypervisor documentation.
To obtain this additional documentation and support for writing host modules,
contact your QNX representative.
How shared memory works
To use shared memory, a client application in a guest or in the hypervisor host
needs:
a mapping of the shared memory region
a hardware interrupt it can use to signal other users of shared memory
regions that this region has been updated
The hypervisor provides the shmem vdev, which implements setting
up the shared memory mapping and the interrupts you need to use shared memory. This
vdev provides additional functionality to simplify using shared memory, including:
shared memory region names (a QNX hypervisor system may include multiple named
shared memory regions)
selective signaling (e.g., signal Guest 0, but not Guest 3)
signal knowledge (the ability to know which guests have been signaled)
Figure 1A 128 MB memory allocation (moo) shared by Guest 0 and Guest 1
The figure above illustrates memory sharing between two guests. Guest 0 attempts to
attach to a 128 MB shared memory area (moo) first. Since no such area exists
at the specified location, the attempt to attach creates the area and allows the
guest to attach to it. Guest 1 can simply attach to the same name to share data
with Guest 0.
Note:
All connections to a shared memory region are peers. That is, there is no
distinction between the guest that creates a shared memory region and the guest
that attaches to it. Simply, the first attempt to attach to a shared memory
region creates the region. As far as the guest is concerned, it simply attaches
to the region.
This design avoids ordering problems where the system designer would have to make
sure that one guest always comes up and creates the shared memory region before
another guest tries to attach to it.