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Primus-vk

This Vulkan layer can be used to do GPU offloading. Typically you want to display an image rendered on a more powerful GPU on a display managed by an internal GPU.

It is basically the same as Primus for OpenGL (https://github.com/amonakov/primus). However it does not wrap the Vulkan API from the application but is directly integrated into Vulkan as a layer (which seems to be the intendend way to implement such logic).

Usage

First you need to install primus_vk:

On Archlinux there are official packages (for 64-bit games, for 32-bit games).
On Debian (from bullseye on) you should use primus-vk-nvidia (which recommends also the 32-bit variants of those packages for 32-bit games), which already is preconfigured for the Nvidia dedicated + Intel integrated graphics setup. When you have a different setup, you should install just primus-vk (which installs only the bare primus_vk-library and no graphics drivers), and install the Vulkan drivers, you need manually.
For Fedora there are unofficial packages.
For Gentoo Linux and affiliates distributions, an unoffical overlay branch is develloped here. This overlay branch supports multilib abi_x86_32 and abi_x86_64 for 32 and 64bit Vulkan applications on Intel/Nvidia Optimus configuration.
For other distributions you will likely need to manually install primus_vk. To run an application with primus_vk prefix the command with pvkrun (which in the easiest case is just ENABLE_PRIMUS_LAYER=1 optirun). So instead of running path/to/application, invoke pvkrun path/to/application instead. You should be able to use pvkrun for all applications, independently of them using Vulkan, OpenGL or both.

By default primus_vk chooses a graphics card marked as dedicated and one not marked as dedicated. If that does not fit on your scenario, you need to specify the devices used for rendering and displaying manually. You can use PRIMUS_VK_DISPLAYID and PRIMUS_VK_RENDERID and give them the deviceIDs from optirun env DISPLAY=:8 vulkaninfo. That way you can force primus_vk to work in a variety of different scenarios (e.g. having two dedicated graphics cards and rendering on one, while displaying on the other).

Idea

Just as the OpenGL-Primus: Let the application talk to the primary display and transparently map API calls so that the application thinks, it renders using the primary display, however the VkDevice (and VkImages) comes from the rendering GPU. When the application wants to swap frames, copy the image over to the integrated GPU and display it there.

Why do we need to copy the Image so often?

As far as I can tell VkImage (and VkMemory) objects may not be shared beween different physical devices. So there is not really another way than using memcpy on the images when memmapped into main memory.

Additionally, only images with VK_IMAGE_TILING_OPTIMAL can be rendered to and presentend and only images with VK_IMAGE_TILING_LINEAR can be mapped to main memory to be copied. So I see no better way than copying the image 3 times from render target to display. On my machine the memcpy from an external device was pretty clearly the bottleneck. So it is not really the copying of the image, but the transfer from rendering GPU into main memory.

An idea might be to use VK_EXTERNAL_MEMORY_HANDLE_TYPE_HOST_ALLOCATION_BIT_EXT to map one device's memory and use that directly on the other device (or import host-allocated memory on both devices). However that is not implemented yet.

Dependencies

This layer requires two working vulkan drivers. The only hardware that I have experience with are Intel Integrated Graphics + Nvidia. However it should theoretically work with any other graphics setup of two vulkan-compatible graphics devices. For the Nvidia graphics card, both the "nonglvd" and the "glvnd" proprietary driver seem to work, however the "nonglvnd"-driver seems to be broken around 430.64 and is removed in newer versions.

To use this layer you will require something similar to bumblebee to poweron/off the dedicated graphics card.

Due to a bug/missing feature in the Vulkan Loader you will need Vulkan/libvulkan >= 1.1.108. If you have an older system you can try primus_vk version 1.1 which contains an ugly workaround for that issue and is therefore compatible with older Vulkan versions.

Development Status

This layer works for all the applications I tested it with, but uses a fair share of CPU resorces for copying.

Technical Limitations

The NVIDIA driver always connect to the "default" X-Display to verify that it has the NV-GLX extensions availible. Otherwise the NVIDIA-vulkan-icd driver disables itself. For testing an intermediate solution is to modify the demo application to always use ":0" and set DISPLAY to ":8" to make the NV-Driver happy. However this approach does work on general applications that cannot be modified. So this issue has to be solved in the graphics driver.
Currently under Debian unstable the nvidia-icd is registered with a non-absolute library path in /usr/share/vulkan/icd.d/nvidia_icd.json. Replace libGL.so.1 with /usr/lib/x86_64-linux-gnu/nvidia/libGL.so.1 there to always load the intended Vulkan driver.
When running an applications with DXVK and wine, wine loads both Vulkan and OpenGL. This creates a problem as:
1. Wine loads Vulkan, which loades the integrated GPU's ICD, the Nvidia ICD (contained in Nvidia's libGL.so on my system), Primus-VK and potentially more.
2. Wine loads OpenGL, which should be satisfied by OpenGL-Primus. However for whatever reason wine directly gets Nvidia's libGL which fails to provide an OpenGL context for the primary X screen. This needs to be prevented by forcing wine to load Primus' libGL.

Issues 1.,2. and 3. can be worked around by compiling libnv_vulkan_wrapper.so and registering it instead of nvidia's libGL.so.1 in /usr/share/vulkan/icd.d/nvidia_icd.json.

Installation

Locally

Create the folder ~/.local/share/vulkan/implicit_layer.d and copy primus_vk.json there with the path adjusted to the location of the shared object.

System-wide

Copy primus_vk.json to /usr/share/vulkan/implicit_layer.d and adjust the path.

Howto

Install the correct vulkan icds (i.e. intel/mesa, nvidia, amd, depending on your hardware).
Use make libprimus_vk.so libnv_vulkan_wrapper.so to compile Primus-vk and libnv_vulkan_wrapper.so (check that the path to the nvidia-driver in nv_vulkan_wrapper.so is correct).
Ensure that the (unwrapped) nvidia driver is not registered (e.g. in /usr/share/vulkan/icd.d/nvidia_icd.json) and create a similar file nv_vulkan_wrapper.json where the path to the driver points to the compiled libnv_vulkan_wrapper.so.
(Optional) Run optirun primus_vk_diag. It has to display entries for both graphics cards, otherwise the driver setup is broken. You can also test with optirun vulkaninfo that your Vulkan drivers are at least detecting your graphics cards.
Install primus_vk.json and adjust path.
Run ENABLE_PRIMUS_LAYER=1 optirun vulkan-smoketest.

Arch Linux

Notes for running on Arch Linux:

nv_vulkan_wrapper.cpp: Change nvDriver path to /usr/lib/libGLX_nvidia.so.0
primus_vk.cpp: add: #include "vk_layer_utils.h" (on Debian the contents are included in some other header and there is no "vk_layer_utils.h")

RPM package

Leonid Maksymchuk built RPM packaging scripts for primus-vk which can be found in his repository. RPMs for Fedora >= 30 are available here

Credits

This layer is based on the sample layer available under https://github.com/baldurk/sample_layer. The guide that goes along with it is https://renderdoc.org/vulkan-layer-guide.html.