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# Graphics Execution
> **Scope.** How the emulated vGPU runs **3D graphics** work — the OpenGL and
> Vulkan command streams a stock guest submits — on the physical GPU, and what
> is proven about it. This is the shared machinery behind
> [`OpenGL.md`](OpenGL.md), [`Vulkan.md`](Vulkan.md) and the Windows/DirectX
> target ([`DirectX.md`](DirectX.md)); CUDA compute uses the same submission and
> completion path for a different engine workload
> ([`Execution-Frontier.md`](Execution-Frontier.md)). Canonical sources:
> [`../docs/04-implementation/05-graphics-execution-plan.md`](../docs/04-implementation/05-graphics-execution-plan.md)
> (the plan and milestone ladder) and
> [`../docs/04-implementation/04-real-render-walls.md`](../docs/04-implementation/04-real-render-walls.md)
> (the measured walls); the docs tree wins on disagreement.
Status: 2026-11-09, tree `master @ 21e0d611`. Tags: `[live]` observed end to
end; `[wip]` partially built; `[wall]` current blocker; `[target]` defined
success state; `[planned]` designed, not built.
## 1. What "real pixels" means
The acceptance target is strict: a **stock** guest GL or Vulkan application's
draw commands are executed by the physical GPU's fixed-function 3D pipeline
(vertex fetch, primitive assembly, rasterisation, fragment work, ROP), the
render target is readable by the guest, and that image reaches the capture
path through the emulated mdev. The pixels must not come from another GPU, a
software rasteriser, or a host display round-trip.
(The project has also published two deliberately-labelled two-VM demo videos
that use the standard `virtio-gpu`/`virgl` virtual GPU on a host integrated
GPU, and a CUDA-animation video that *does* run on the emulated vGPU through
the compute path. The `virgl` videos are honest demonstrations of the guest
experience; they are not this execution path, and their captions say so.)
## 2. The path a graphics command takes
```mermaid
graph TD
APP["Guest applications<br/>OpenGL · Vulkan · CUDA"] --> LIB["Guest user space<br/>GL: GLX client + the driver's GL core<br/>Vulkan: the NVIDIA ICD<br/>CUDA: libcuda"]
APP --> X["X server (Linux GL)<br/>GLX module + the NVIDIA DDX"]
LIB --> KMD["Guest kernel: nvidia.ko<br/>RM core, GSP client, channels, modeset/drm"]
X --> KMD
KMD -- "GPFIFO rings + doorbell;<br/>method streams in push buffers" --> VG["Host module vgsp.ko<br/>device model + emulation core"]
VG --> L1["Lane 1: verbatim 3D<br/>method-stream replay"]
VG --> L2["Lane 2: synthesized launch<br/>from a captured section"]
VG --> L3["Lane 3: forwarded CUDA<br/>launch (compute)"]
L1 --> FE["Physical GPU<br/>front-end fetch -> GR engine"]
L2 --> FE
L3 --> FE
FE -- "engine-authored releases<br/>(semaphores in guest memory)" --> VG
VG -- "completion relay: MSI-X +<br/>poller/event wake the guest" --> KMD
```
```text
guest apps (GL / Vulkan / CUDA)
-> guest user space (GLX client + GL core, Vulkan ICD, libcuda)
-> guest X server (GLX module + NVIDIA DDX) [OpenGL]
-> nvidia.ko: channels, GPFIFO rings, doorbells [all APIs]
-> vgsp.ko: capture sections, decode pushes, forward
-> physical GPU front-end -> GR engine (3D + compute) / copy engines
<- engine-authored releases written into guest memory
<- completion relay (MSI-X + wakes) back to the guest
ASCII fallback above; the diagram shows the three execution lanes.
```
## 3. The three execution lanes
Graphics work reaches the engines today in one of three shapes, in increasing
fidelity:
1. **Verbatim 3D method-stream replay** `[live]`. The emulation captures the
X server's graphics-class (`0xC597`, `TURING_A`) section from its channel,
relocates the operand addresses into host-built shadow pages the engine can
reach, and submits the stream unchanged. The engine retires it: the uploads
(`LINE_LENGTH_IN`/`LAUNCH_DMA`), the state, program and texture binds, and
the release all execute with no MMU fault. This proves the *mechanics* of
3D execution — object bind, window placement, operand relocation,
completion — on real hardware.
2. **Synthesized launch from a captured section** `[live]`. A compute-shaped
launch built from the captured section (operand list, program, a 64-word
launch descriptor) reaches the engine and retires real `sm_75` SASS; the
Turing shader-program-header form (`V3`) is accepted with zero header
faults across repeated launches. This proved engine-side dispatch before
the verbatim lane existed. The program itself is still a development
stand-in at this rung — execution proof, not pixels.
3. **Forwarding the client's own pushes** `[wip]`. The GL client's own
channels now register, submit and retire: the direct-GL GR channel
(`0xbeef0100`) and its CE siblings (`0xbeef0101`/`0xbeef0102`) are
discovered, their rings and releases are decoded, and the X server's DDX
kick channel (`0xbfef0002`) is decoded from its ring head with its
class-2D notification authored. The X DDX waits are fixed (class-aware
display arm, `0x8000 -> 0` notification completion, decoded GL release
applied), and with GLX forced (`SDL_VIDEO_X11_FORCE_EGL=0`) the windowed
GL path selects `libnvidia-glcore`. What remains is the kernel-visible GL
present/kick completion, the DDX get-word/notifier recycle, and physical
dispatch of the client's draws (§6).
## 4. The channel and object picture
* **Every submission lives on a channel**: a GPFIFO ring plus engine objects,
with a USERD the driver advances its put pointer in and a doorbell the host
module traps. Channels are typed by engine: a graphics channel carries 3D
state and draws; copy-class channels carry memory movements.
* **The graphics object class** the guest's 3D stream binds is the Turing
graphics class, `0xC597`. Streams now come from both the **X server's
channel** and the **direct GL client's own channels** (`0xbeef0100` GR plus
the `0xbeef0101`/`0xbeef0102` CE siblings); the 2D present/readback work
rides lighter 2D-class and copy-engine shapes.
* **Display and modeset** `[live]`. The guest's modeset path is served by an
emulated display engine: the display push-buffer channels (core `0xc57d`,
window `0xc57e`, immediate/window `0xc57b`) are registered, their push
buffers parsed, and the notifier/`UPDATE` completion authored. The arm is
class-aware (the window class has no NOTIFY bit; a bound notifier is the
arm), and the X DDX's `0x8000 -> 0` notification completion is authored.
This is what lets nvidia-modeset/NvKms register (`/dev/dri`), X settle and
GLX clients obtain a real GL context. The display-capability seam is
parameterised in the campaign configuration.
* **Cells of state**: 3D streams are long—hundreds of method words per draw —
and mix state binds (viewport, render target, ZS, blend/ROP), program and
constant binds, vertex/index streams and draw-issuing methods. The capture
machinery recognises the init-shaped operands today; the per-draw state
vocabulary is being extended ([G-B in the plan](
../docs/04-implementation/05-graphics-execution-plan.md)).
* **Synchronisation**: tracking semaphores, notifications and fence values
written by the engine; the guest's swap/present waits sit on top of those.
## 5. Completion, fences and presents
The acceptance signal is the **engine-authored release**: a semaphore the
engine writes when the work completes. For GL, completion flows through the
channel's semaphores/notifications to the X server and the app's swap wait;
for Vulkan, the client's fence value is released; for CUDA, the launch's
release group wakes the client. The emulation relays engine completions into
guest-observable memory (MSI-X plus a poller), and the guest resumes from
there.
For the display path, completion is the notifier write and `UPDATE ->
FINISHED`; a completion event deferred by the reply path is force-retried
until it posts, so the X server's mode set finishes. For the X DDX's own 2D
push buffers, completion also requires the GPU-written engine get word to
advance and the notifier to be recycled to `0xffff` after a kick; both are
host-side levers now, and a diagnostic write of the get word released the
DDX's reserve loop. For a windowed GL client the present is a kick whose
completion the guest kernel observes by polling its per-slot
`/dev/nvidia-modeset` fd; the host must complete the exact notifier page the
kernel watches and signal that event, not only write the notifier memory.
During bring-up the module also carries a **diagnostics-only mirror** for apps
that spin on a shared progress word; it exists so a run can proceed while the
natural completion path is being finished. The 60-fps milestone explicitly
requires it switched off, along with any external poke.
## 6. Where the frontier is (ranked walls)
1. **Kernel-visible GL present completion** `[wall]`. With GLX forced the
windowed client uses `libnvidia-glcore`; the minimal `glxgears` then hangs
in the glcore kick wait: the client polls its per-slot
`/dev/nvidia-modeset` fd (`poll(POLLIN, 1 s)`). The host's sweep completes
the beef notifier memory, but the guest kernel never signals the event the
poll waits on. The exact notifier page the kernel watches must be
completed and the guest event signalled (the kick-notify executor), not
just the memory written.
2. **The X DDX push-buffer completion** `[wall]`. The DDX's 2D reserve loop
exits only when the GPU-written engine get word advances (`get <= put`) and
the kick requires the notifier to be recycled to `0xffff` after
completion. A diagnostic host write released the loop; the kernel levers
to publish/advance the get word and recycle the notifier are next.
3. **The X DDX footprint** `[wall]`. The DDX allocates 4 GB of system pages at
startup; an 8 GB guest has no headroom once GZDoom's own footprint lands
and both are OOM-killed. Current runs use a 12 GB guest; the allocation is
a suspected emulation gap (a 512 MiB emulated VRAM falling back to a 4 GB
sysmem allocation) to fix.
4. **Physical dispatch of the GL draws** `[wall]`. The direct-GL channels
submit and get host-completed releases, but the forwarding path reports no
dispatch to the physical GR engine (`forwarded=0`); the draws are decoded
and host-completed, not executed on hardware.
5. **Non-black frames and sustained rate** `[wall]`/`[target]`. Once the
kernel-visible present completion and the DDX get-word/notifier completion
land, the capture must show motion, then 60 fps in one VM with no forcing,
then two VMs on two mediated devices with non-black frames in both.
## 7. What is proven today
| Capability | State |
|---|---|
| X server initialises on the emulated vGPU (driver DMA sanity push, ScreenInit) | `[live]` |
| Display engine serves the modeset push buffers; notifier plus `UPDATE -> FINISHED` | `[live]` (campaign configuration) |
| nvidia-modeset/NvKms registers (`/dev/dri/card0`, `renderD128`, `fb0`); `nvidia-smi -L` enumerates | `[live]` |
| `glxinfo` reports the emulated GPU with the NVIDIA renderer (OpenGL 4.6.0, direct rendering) | `[live]` |
| GZDoom obtains a real GL context; windowed reaches renderer startup | `[live]` (pixels frontier) |
| GZDoom reaches `MAP01` on Mesa llvmpipe (not the emulated vGPU path) | `[wip]` |
| Forced GLX (`SDL_VIDEO_X11_FORCE_EGL=0`) selects `libnvidia-glcore` | `[live]` |
| Windowed GL present/kick completion (minimal `glxgears`) | `[wall]` |
| X server survives the GZDoom run on a 12 GB guest (the DDX's 4 GB sysmem allocation forces the size) | `[live]` |
| GL section capture: the X server's `0xC597` stream decoded | `[live]` |
| Verbatim 3D method-stream replay retired by the engine (no MMU fault) | `[live]` |
| Synthesized launch executes real `sm_75` SASS; Turing `V3` program header accepted | `[live]` |
| 2D screen fills and copy-engine present ops execute | `[live]` |
| Direct GL client: device-init gate handled, channels allocate, readback path returns clean | `[live]` |
| Direct GL client channels submit and retire (GR `0xbeef0100`, CE `0xbeef0101`/`0xbeef0102`); DDX kick channel decoded | `[live]` |
| Guest kernel observes the kick completion and signals the modeset event | `[wall]` |
| NVIDIA Vulkan ICD creates a device | `[wall]` |
| DDX push-buffer get-word publish and notifier recycle | `[wall]` |
| Physical dispatch of the direct-GL draws (`forwarded=0`) | `[wall]` |
| Non-black pixels from the emulated vGPU's 3D pipeline | `[wall]` |
| Windows/DirectX guests | `[planned]` |
## 8. What is not claimed
* No non-black pixel has been produced by the emulated vGPU's 3D pipeline yet.
* Display/KMS bring-up (NvKms, `/dev/dri`, X, a real GL context) is not a
rendered-frame claim; GZDoom reaching `MAP01` on Mesa llvmpipe is not the
emulated-vGPU path, and the later forced-GLX run selects `libnvidia-glcore`
but hangs in the present/kick wait. The kernel-visible present completion,
the DDX get-word/notifier completion and the physical dispatch of the GL
draws are the current walls.
* No frame-rate or latency numbers: the scheduled measurement is defined
(60 fps over 60 s, no forcing) but not yet met.
* The `virgl`-based demo videos are a different, labelled path — not this one.
## 9. Related pages
* [`OpenGL.md`](OpenGL.md) — the guest OpenGL path and its status.
* [`Vulkan.md`](Vulkan.md) — the guest Vulkan path and its status.
* [`DirectX.md`](DirectX.md) — Windows guests and the DirectX target.
* [`Execution-Frontier.md`](Execution-Frontier.md) — the CUDA compute chain,
proven end to end.
* [`Physical-GPU-Backend.md`](Physical-GPU-Backend.md) — the backend the work
is forwarded to.
* [`Introduction-to-GMMU.md`](Introduction-to-GMMU.md) — channels, GPFIFO,
engines, and page tables.
* [`Host-Module.md`](Host-Module.md) — where capture, decode and forwarding
live.
* [`../docs/04-implementation/05-graphics-execution-plan.md`](../docs/04-implementation/05-graphics-execution-plan.md) —
the canonical plan: gap list, three approaches, and the M-GL1..M-GL6
milestone ladder with gates.
|
