# Real-render walls: Vulkan and Xorg on the emulated vGPU

Status: OPEN (2026-10-29, SESSION 64). This page records the measured state
of the guest's *display/graphics* consumers - the Vulkan ICD and the Xorg
server - on the emulated vGPU, with the exact evidence and the next
subsystem to implement. It complements
[05-operations/06-guest-frame-path-and-looking-glass.md](../05-operations/06-guest-frame-path-and-looking-glass.md)
(which covers the CUDA -> ivshmem -> host frame transport that is already
proven).

## 1. What works today

* The guest's shipped 570.144 Vulkan userspace **loads and enumerates**:
  `vkCreateInstance` succeeds and two physical devices are reported -
  `NVIDIA GeForce RTX 2080 SUPER` (discrete, Vulkan 1.4.303) and the
  software `llvmpipe` - from `/etc/vulkan/icd.d/nvidia_icd.json`. Probes:
  `tools/vk/vk_probe.py`, `tools/vk/vk_probe2.py`.
* `eglInitialize` succeeds with `vendor=NVIDIA` on the guest EGL device
  platform (the EGL probe `tools/guest-diag/egl_retry.c` and the older
  `egl_frame`).
* CUDA is unaffected (its clients run through the same emulated GSP; see
  the battery evidence in the handoff).

## 2. Wall 1 - `vkCreateDevice` fails in nvidia-modeset's ALLOC_DEVICE

`vkCreateDevice` returns `VK_ERROR_INITIALIZATION_FAILED`. An
`LD_PRELOAD` ioctl logger (`tools/guest-diag/nvkmstrace.c`) shows the
ICD's first nvKms ioctl failing:

```
NVKMS_IOCTL cmd=0 size=1488 addr=0x... rc=-1   # NVKMS_IOCTL_ALLOC_DEVICE
```

i.e. the kernel's `nvKmsAllocDevice` (in `nvidia-modeset`) returned a
non-success status; the userspace frees everything and reports the
generic initialization error. Reading the driver's own modeset source
(`nvkms-rm.c`) gives the alloc order and the two gates we hit and fixed:

1. `ProbeHeadCountAndWindowAssignment()` -> `NV0073_CTRL_CMD_SYSTEM_GET_NUM_HEADS`
   (`0x730102`): a zero reply logs `No heads found on board!` and fails the
   alloc. The emulated GSP RSP answered all NV0073 controls through the
   generic zero-reply path.
2. After the heads: `NoValidDisplays()` -> `NV0073_CTRL_CMD_SYSTEM_GET_SUPPORTED`
   (`0x730107`) must report at least one display id, and the connector
   path (`SPECIFIC_OR_GET_INFO` `0x73028b`, `SPECIFIC_GET_CONNECTOR_DATA`
   `0x730250`, `DFP_GET_INFO` `0x731140`) must describe a connector.

The host GSP-RM emulation now implements a minimal headless display:
one head (`0x730102`/`0x730287`), flexible window assignment (`0x7302ad`),
one static HDMI connector on display id 0 (SOR + single-link TMDS, which
avoids the DP-AUX library), plus DDC support. With this shim the alloc
gets past heads and `NoValidDisplays` and proceeds through the connector
queries and the display-object allocations (classes `0xc570`, `0xc57d`,
`0xc372`, controls `0x50700709`, `0x73010c`, `0xc3700201`) before failing
later in the display-core/modeset-channel path. That remaining subsystem
is the wall: the modeset core channel and the per-display dpy objects
must be emulated (a coupled object + channel + memory set whose exact
wire forms should be captured from a real-GPU session).

A related experiment ruled out one shortcut: booting the guest with
`nvidia-modeset`/`nvidia-drm` blacklisted (no `/dev/nvidia-modeset`) does
not avoid the wall - the ICD's nvKms client still issues ALLOC_DEVICE and
fails, so the display path cannot be sidestepped by disabling KMS.

## 3. Wall 2 - Xorg ScreenInit fails in nvidia-push's channel test

Xorg starts (the GPU is detected, `Virtual screen 1280x1024`) and then:

```
(EE) NVIDIA(GPU-0): Failed to initialize DMA.
(EE) NVIDIA(0): Failed to allocate push buffer
```

`nvidia-push` logs `Failed to initialize DMA.` from
`__nvPushTestPushBuffer()`: right after allocating a push buffer the
driver writes a NOP and waits (10 s) for the channel to consume it. Host
evidence shows why it never does:

* the X driver's channel registers (`class 0xc46f` GPFIFO channel,
  `gpfifo=0x120090000`, `userd=0x350000` sysmem) and lives with
  **`submissions=0`** - its published GP_PUT never advances in the host's
  view, because the host needs the channel's VA->PA translations to read
  its USERD/GPFIFO and that resolution comes from the guest-side scanner
  that currently follows *CUDA clients only* (`targeted resolve
  va=0x20400f000: full RAM scan found nothing`);
* on a busy boot (Xorg started while a CUDA battery ran) the channel's
  completion-counter slot is rejected: `counter auto h=0x1000013 rank=0
  pa=0x11c827ff0 rejected (slot owned by another channel)` - two
  concurrent clients both derive rank 0's slot from the per-device
  structural formula.

Fixing Xorg therefore needs (1) a platform-wide (non-CUDA) VA-resolution
path for the scanner or the host, and (2) counter-slot ownership keyed by
client identity (or a per-client slot domain), not just rank.

## 4. Wall 3 - the EGL/glsi engine-init gate (unchanged)

`eglMakeCurrent` returns `EGL_BAD_ALLOC (0x3003)`; the glsi engine-init
entry returns a "busy" status once, sets a state bit, and the eglcore
caller does not retry. Ten in-process retries (`egl_retry`) all fail, so
the state machine fails on a real input - the return-value trace of its
callees remains the way in (see
[userspace/cuda/570.144/docs/ANALYSIS-egl-engine-selection.md](../../userspace/cuda/570.144/docs/ANALYSIS-egl-engine-selection.md)).

## 5. Why this matters for the frame path

Everything downstream of a working GL/Vulkan context is already in place:
a guest publisher writes frames into the kvmfr ivshmem BAR
(`tools/vk/vk_frame.c`, `tools/guest-diag/xshm_pub.c` use the same layout
as the proven CUDA `frame_producer`), the host reader decodes them
(`tools/lg/frame_reader.py`) and ffmpeg assembles video. The missing
piece is a *working* graphics API on the guest GPU - the two subsystems
above.