RFC: New device for zero-copy VM memory access

[geoff@hostfission.com]

Hi All,

Over the past week, I have been working to come up with a solution to 
the
memory transfer performance issues that hinder the Looking Glass 
Project.

Currently Looking Glass works by using the IVSHMEM shared memory device 
which
is fed by an application that captures the guest's video output. While 
this
works it is sub-optimal because we first have to perform a CPU copy of 
the
captured frame into shared RAM, and then back out again for display. 
Because
the destination buffers are allocated by closed proprietary code 
(DirectX, or
NVidia NvFBC) there is no way to have the frame placed directly into the
IVSHMEM shared ram.

This new device, currently named `introspection` (which needs a more 
suitable
name, porthole perhaps?), provides a means of translating guest physical
addresses to host virtual addresses, and finally to the host offsets in 
RAM for
file-backed memory guests. It does this by means of a simple protocol 
over a
unix socket (chardev) which is supplied the appropriate fd for the VM's 
system
RAM. The guest (in this case, Windows), when presented with the address 
of a
userspace buffer and size, will mlock the appropriate pages into RAM and 
pass
guest physical addresses to the virtual device.

This device and the windows driver have been designed in such a way that 
it's a
utility device for any project and/or application that could make use of 
it.
The PCI subsystem vendor and device ID are used to provide a means of 
device
identification in cases where multiple devices may be in use for 
differing
applications. This also allows one common driver to be used for any 
other
projects wishing to build on this device.

My ultimate goal is to get this to a state where it could be accepted 
upstream
into Qemu at which point Looking Glass would be modified to use it 
instead of
the IVSHMEM device.

My git repository with the new device can be found at:
https://github.com/gnif/qemu

The new device is:
https://github.com/gnif/qemu/blob/master/hw/misc/introspection.c

Looking Glass:
https://looking-glass.hostfission.com/

The windows driver, while working, needs some cleanup before the source 
is
published. I intend to maintain both this device and the windows driver
including producing a signed Windows 10 driver if Redhat are unwilling 
or
unable.

Kind Regards,
Geoffrey McRae

HostFission
https://hostfission.com