ai-app/rigs/virtgpu-probe/virtgpu-probe.c

// What this VM's virtio-gpu actually offers, asked of the kernel and the
// driver rather than inferred from the host's qemu command line.
//
//     cc -O2 -o virtgpu-probe virtgpu-probe.c -I/usr/include/libdrm -ldrm -lvulkan
//     ./virtgpu-probe
//
// Written 2026-09-08 for the question "Venus keeps causing problems, is
// there something to do with qemu instead" (docs/RUST.md, "Venus went
// away for an hour"). Three things, each of which was guessed wrong at
// least once before being measured:
//
//   1. Which capsets the host offers. Capset 6 (DRM) is the "native
//      context" one -- RADV running in the guest against a passed-through
//      DRM context instead of Venus proxying every Vulkan call. Whether
//      it is available is a host-side fact this is the only way to read
//      from in here.
//   2. Whether the Vulkan device has compute. It does, and assuming it
//      did not sent one investigation down the wrong path: the "no
//      compute" finding on record is about the *Android emulator's*
//      SwiftShader GL path, a different machine entirely.
//   3. Whether plain Vulkan device teardown crashes on this adapter. It
//      does not -- which is what makes wgpu's teardown SIGSEGV wgpu's and
//      not the driver's, and is the kind of claim that is worthless
//      without the negative half.
//
// C rather than a Rust crate on purpose: two of the three are ioctl and
// loader questions that a wgpu-shaped rig cannot ask, and this needs to
// keep working when the thing under suspicion is wgpu itself.
#include <errno.h>
#include <fcntl.h>
#include <stdint.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>

#include <drm/virtgpu_drm.h>
#include <vulkan/vulkan.h>
#include <xf86drm.h>

static const char *capset_name(int id) {
    switch (id) {
    case 1: return "VIRGL";
    case 2: return "VIRGL2";
    case 3: return "GFXSTREAM_VULKAN";
    case 4: return "VENUS";
    case 5: return "CROSS_DOMAIN";
    case 6: return "DRM (native context)";
    default: return "unknown";
    }
}

static int capsets(void) {
    int fd = open("/dev/dri/renderD128", O_RDWR);
    if (fd < 0) {
        printf("capsets: cannot open /dev/dri/renderD128: %s\n", strerror(errno));
        return 1;
    }
    struct drm_virtgpu_getparam gp;
    uint64_t mask = 0;
    memset(&gp, 0, sizeof gp);
    gp.param = VIRTGPU_PARAM_SUPPORTED_CAPSET_IDs;
    gp.value = (uint64_t)(uintptr_t)&mask;
    int rc = drmIoctl(fd, DRM_IOCTL_VIRTGPU_GETPARAM, &gp);
    close(fd);
    if (rc) {
        // Not a virtio-gpu at all, or a kernel without the param: say
        // which, rather than printing an empty list that reads like "the
        // host offers nothing".
        printf("capsets: SUPPORTED_CAPSET_IDs unavailable: %s\n", strerror(errno));
        return 1;
    }
    printf("capsets: bitmask 0x%llx\n", (unsigned long long)mask);
    for (int i = 1; i <= 8; i++)
        if (mask & (1ull << i)) printf("  %d: %s\n", i, capset_name(i));
    if (!(mask & (1ull << 6)))
        printf("  (no capset 6: native context needs host-side virglrenderer + qemu support)\n");
    return 0;
}

static int devices(void) {
    VkInstanceCreateInfo ici = {.sType = VK_STRUCTURE_TYPE_INSTANCE_CREATE_INFO};
    VkInstance inst;
    if (vkCreateInstance(&ici, NULL, &inst) != VK_SUCCESS) {
        printf("vulkan: no instance -- the loader found no usable ICD\n");
        return 1;
    }
    uint32_t n = 0;
    vkEnumeratePhysicalDevices(inst, &n, NULL);
    if (n == 0) {
        // The exact state this VM was in for an hour on 2026-09-08.
        printf("vulkan: instance ok but ZERO devices -- the host refused a context\n");
        vkDestroyInstance(inst, NULL);
        return 1;
    }
    VkPhysicalDevice pd[8];
    if (n > 8) n = 8;
    vkEnumeratePhysicalDevices(inst, &n, pd);
    for (uint32_t i = 0; i < n; i++) {
        VkPhysicalDeviceProperties p;
        vkGetPhysicalDeviceProperties(pd[i], &p);
        printf("vulkan: %s (api %u.%u.%u)\n", p.deviceName, VK_VERSION_MAJOR(p.apiVersion),
               VK_VERSION_MINOR(p.apiVersion), VK_VERSION_PATCH(p.apiVersion));
        printf("  compute: %u invocations/workgroup, size %u,%u,%u, %u bytes shared\n",
               p.limits.maxComputeWorkGroupInvocations, p.limits.maxComputeWorkGroupSize[0],
               p.limits.maxComputeWorkGroupSize[1], p.limits.maxComputeWorkGroupSize[2],
               p.limits.maxComputeSharedMemorySize);
    }

    // The negative half of "wgpu's teardown crashes on Venus": five
    // devices and the instance, created and destroyed the plain way.
    float prio = 1.0f;
    VkDeviceQueueCreateInfo q = {.sType = VK_STRUCTURE_TYPE_DEVICE_QUEUE_CREATE_INFO,
                                 .queueFamilyIndex = 0, .queueCount = 1, .pQueuePriorities = &prio};
    VkDeviceCreateInfo dci = {.sType = VK_STRUCTURE_TYPE_DEVICE_CREATE_INFO,
                              .queueCreateInfoCount = 1, .pQueueCreateInfos = &q};
    for (int i = 0; i < 5; i++) {
        VkDevice dev;
        if (vkCreateDevice(pd[0], &dci, NULL, &dev) != VK_SUCCESS) {
            printf("teardown: device %d could not be created\n", i);
            return 1;
        }
        vkDestroyDevice(dev, NULL);
    }
    vkDestroyInstance(inst, NULL);
    printf("teardown: 5 devices + instance created and destroyed cleanly\n");
    return 0;
}

int main(void) {
    int bad = capsets();
    bad |= devices();
    return bad;
}