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

[Gerd Hoffmann <kraxel@redhat.com>]

  Hi,

> 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.

So, if I understand things correctly, the workflow looks like this:

  (1) guest allocates buffers, using guest ram.
  (2) guest uses these buffers as render target for the gpu (pci-assigned I guess?).
  (3) guest passes guest physical address to qemu (via porthole device).
  (4) qemu translates gpa into file offset and passes offsets to
      the client application.
  (5) client application maps all guest ram, then uses the offsets from
      qemu to find the buffers.  Then goes displaying these buffers I guess.

Correct?

Performance aside for now, is it an option for your use case to simply
use both an emulated display device and the assigned gpu, then configure
screen mirroring inside the guest to get the guest display scanned out
to the host?

cheers,
  Gerd