visor/pkg/sentry/devices/nvproxy

nvproxy

The nvproxy package is a core component of gVisor that enables support for NVIDIA GPUs, allowing sandboxed applications to perform GPU-accelerated computations. This is achieved by intercepting and forwarding NVIDIA driver calls from the sandboxed application to the host's NVIDIA driver.

How it Works

The nvproxy driver operates by implementing virtual character devices within the gVisor sandbox that mimic actual NVIDIA device files (like /dev/nvidiactl, /dev/nvidia-uvm and /dev/nvidia#). When an application inside the sandbox opens and interacts with these devices, nvproxy intercepts the ioctl and mmap system calls. These calls, which are typically used for communication with the NVIDIA driver, are then forwarded to the actual host NVIDIA driver after necessary translations.

This proxying mechanism allows gVisor to maintain a strong security boundary while still providing applications with access to the powerful computational capabilities of the GPU. All other system calls from the application continue to be handled by the gVisor Sentry.

For more information about gVisor GPU support, see the user guide.

Adding Support for New Driver Versions

The nvproxy package is sensitive to changes in the NVIDIA driver's Application Binary Interface (ABI), which can occur between driver releases. This mainly happens when ioctl(2) structs are modified. To manage this, nvproxy is designed to support multiple driver versions explicitly.

This is accomplished using a sparse version tree defined in version.go. This tree doesn't list every NVIDIA driver release; instead, it only contains the specific versions required to model the ABI's evolution across all supported versions.

The tree's structure mimics the commit history of NVIDIA kernel driver repo, including releases from both the master branch and separate development branches. This is critical because ABI changes introduced in a parent version affect all subsequent child versions. An accurate tree allows nvproxy to correctly compose the final ABI for any given version.

At runtime, nvproxy performs the following steps:

1. Reads the host's NVIDIA driver version.
2. Finds the corresponding version in its tree.
3. Traverses the tree from the root to that version's node, applying all ABI modifications along the path.

Here is the step-by-step process for adding support for a new driver version.

Step 1: Place the New Version in the Tree

First, determine the new version's correct position in nvproxy's version tree.

1. Find the parent commit: Go to the NVIDIA driver releases page and find the commit hash for your target version. Traverse the commit history upwards (following parent links) until you find a version that already exists in nvproxy's version tree.
2. Insert the new version: Add the new version to the tree under its identified parent.
3. Mimic the branch structure: If the new version was created on a separate development branch (i.e., not master), you must replicate that branch structure in the nvproxy tree. If the branch point is a version nvproxy doesn't officially support, add it as an "unqualified" node (a version without a checksum or official support) to maintain structural integrity.

Step 2: Calculate the Driver Checksum

The version tree requires a SHA256 checksum of the official NVIDIA driver installer (.runfile) for verification. You can calculate this using the provided tool:

bazel run tools/gpu:main checksum -- --version=<DRIVER_VERSION>

Step 3: Account for ABI Changes

Use our nvidia_driver_differ tool to detect changes to proxied ABI structs between the parent and the new version. The tool analyzes the NVIDIA kernel driver source code and outputs the impacted structs.

bazel run tools/nvidia_driver_differ:run_differ -- --base <PARENT_VERSION> --next <NEW_VERSION>

• <PARENT_VERSION> is the version of the parent node in nvproxy's version tree.
• <NEW_VERSION> is the version you are adding.

Warning: This tool is for assistance and does not guarantee completeness. You must still perform manual verification and testing. GPU tests are run against all supported driver versions during Buildkite presubmits.

To verify changes in nvproxy, you can run the nvproxy_driver_parity_test test, which compares nvproxy's struct definitions with driver struct definitions:

bazel test pkg/sentry/devices/nvproxy:nvproxy_driver_parity_test

Handling Intermediate Versions

It is crucial to introduce ABI changes at the exact version they appear in the driver source, even if nvproxy doesn't officially support that intermediate version. When you identify an ABI struct change, go to its source code in NVIDIA kernel driver repo and see which commit introduced the change (using Blame view).

For example, imagine nvproxy supports version [A] and you want to add support for [C]. However, an ABI change that affects nvproxy was introduced in an intermediate version B.

Incorrect:
Do not apply the changes from B directly into [C].

Correct:
Create an intermediate, unqualified node for B that contains the necessary code changes. The new node for [C] can then inherit these changes from B. This ensures the version history is accurate.

This approach is essential for long-term maintainability. If you later need to support another version [D] that also branched from B, it can accurately inherit the same changes.

[A] -> B -> [C]
        \
         -> [D]

Step 4: Add Support for New or Modified Ioctls

After running the nvidia_driver_differ tool, you may need to add or update ioctl command handlers. To do this correctly, you must find the ioctl's implementation in the NVIDIA kernel driver source code to understand its function and data structures.

The implementation details depend on the ioctl type:

• Frontend Ioctls (/dev/nvidiactl or /dev/nvidia#):
  • For top level commands, see documentation in frontendFD.Ioctl() in frontend.go.
  • For sub-commands of NV_ESC_RM_ALLOC (allocation classes): See documentation in rmAlloc() in frontend.go.
  • For sub-commands of NV_ESC_RM_CONTROL (control commands): See documentation in rmControl() in frontend.go.
• UVM Ioctls (/dev/nvidia-uvm): These require manual implementation by studying the kernel driver source and replicating the logic within nvproxy.

Handling File Descriptors and Pointers

A critical responsibility of nvproxy is to translate file descriptors (FDs) and pointers within ioctl data structures.

• File Descriptors: FDs used by the sandboxed application are local to the gVisor Sentry's FD table. They must be translated to the corresponding host FDs before being passed to the host driver.
• Pointers: Pointers in ioctl structs are virtual addresses within the sandboxed application's memory space. These are invalid on the host. The structs containing them must be copied from the application's memory into the Sentry's memory. The host ioctl call must then be made using a pointer to this Sentry-managed memory.

Simple Ioctls

If an ioctl data structure contains neither pointers nor FDs and has no special mmap semantics, it requires no translation and is considered "simple". Helper utilities exist in nvproxy to proxy these simple ioctls directly, which you should use whenever possible. Majority of ioctls proxied today are simple.