Re: RFC: New device for zero-copy VM memory access

[Dr. David Alan Gilbert]

* address@hidden (address@hidden) wrote:
> 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.

Hi Geroggrey,
  I wonder if the same thing can be done by using the existing vhost-user
mechanism.

  vhost-user is intended for implementing a virtio device outside of the
qemu process; so it has a character device that qemu passes commands down
to the other process, where qemu mostly passes commands via the virtio
queues.   To be able to read the virtio queues, the external process
mmap's the same memory as the guest - it gets passed a 'set mem table'
command by qemu that includes fd's for the RAM, and includes base/offset
pairs saying that a particular chunk of RAM is mapped at a particular
guest physical address.

  Whether or not you make use of virtio queues, I think the mechanism
for the device to tell the external process the mappings might be what
you're after.

Dave

> 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
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK