fix: install absent or incomplete managed Ubuntu GPU drivers

parts/linux/cloud-init/artifacts/cse_config_gpu.sh

@@ -108,7 +108,15 @@ configGPUDrivers() {
 }
 
 validateGPUDrivers() {
-    retrycmd_if_failure 24 5 25 nvidia-modprobe -u -c0 && echo "gpu driver loaded" || configGPUDrivers || exit $ERR_GPU_DRIVERS_START_FAIL
+    # A cache-only VHD has no host driver yet. Retrying a missing executable cannot make it ready
+    # and consumes 115 seconds of the CSE budget before the existing full-install fallback.
+    if [ "$OS" = "$UBUNTU_OS_NAME" ] && [ "${IS_VHD,,}" = "true" ] &&
+        [ "$(getCPUArch)" = "amd64" ] && ! command -v nvidia-modprobe >/dev/null 2>&1; then
+        echo "NVIDIA driver is not installed in this Ubuntu VHD; installing before validation"
+        configGPUDrivers || exit $ERR_GPU_DRIVERS_START_FAIL
+    else
+        retrycmd_if_failure 24 5 25 nvidia-modprobe -u -c0 && echo "gpu driver loaded" || configGPUDrivers || exit $ERR_GPU_DRIVERS_START_FAIL
+    fi
 
     if which nvidia-smi; then
         SMI_RESULT=$(retrycmd_if_failure 24 5 30 nvidia-smi)

spec/parts/linux/cloud-init/artifacts/cse_config_gpu_spec.sh

@@ -74,6 +74,103 @@ Describe 'cse_config_gpu.sh'
         End
     End
 
+    Describe 'validation on cache-only Ubuntu VHDs'
+        OS="$UBUNTU_OS_NAME"
+        IS_VHD=true
+        MOCK_CPU_ARCH=amd64
+        MOCK_MODPROBE_PRESENT=1
+        MOCK_MODPROBE_RESULT=0
+        MOCK_INSTALL_RESULT=0
+        ERR_GPU_DRIVERS_START_FAIL=84
+
+        getCPUArch() { echo "$MOCK_CPU_ARCH"; }
+        command() {
+            if [ "$*" = "-v nvidia-modprobe" ]; then
+                return "$MOCK_MODPROBE_PRESENT"
+            fi
+            builtin command "$@"
+        }
+        retrycmd_if_failure() {
+            echo "retry $*" >&2
+            if [ "$4" = "nvidia-modprobe" ]; then
+                return "$MOCK_MODPROBE_RESULT"
+            fi
+        }
+        configGPUDrivers() {
+            echo "normal driver installation"
+            return "$MOCK_INSTALL_RESULT"
+        }
+        which() { return 0; }
+
+        It 'installs an absent driver without retrying the missing executable'
+            When run validateGPUDrivers
+            The status should be success
+            The output should include "installing before validation"
+            The output should include "normal driver installation"
+            The output should include "gpu driver working fine"
+            The stderr should not include "nvidia-modprobe"
+            The stderr should include "retry 24 5 30 nvidia-smi"
+        End
+
+        It 'accepts the existing uppercase VHD flag'
+            IS_VHD=TRUE
+            When run validateGPUDrivers
+            The status should be success
+            The output should include "normal driver installation"
+            The stderr should not include "nvidia-modprobe"
+            The stderr should include "nvidia-smi"
+        End
+
+        It 'propagates installation failure without reporting successful validation'
+            MOCK_INSTALL_RESULT=1
+            When run validateGPUDrivers
+            The status should equal 84
+            The output should include "normal driver installation"
+            The output should not include "gpu driver working fine"
+            The stderr should equal ""
+        End
+
+        It 'keeps readiness retries for a present driver'
+            MOCK_MODPROBE_PRESENT=0
+            When run validateGPUDrivers
+            The status should be success
+            The output should include "gpu driver loaded"
+            The output should not include "normal driver installation"
+            The stderr should include "retry 24 5 25 nvidia-modprobe -u -c0"
+            The stderr should include "nvidia-smi"
+        End
+
+        It 'preserves the full-install fallback after an installed driver fails to load'
+            MOCK_MODPROBE_PRESENT=0
+            MOCK_MODPROBE_RESULT=1
+            When run validateGPUDrivers
+            The status should be success
+            The output should include "normal driver installation"
+            The output should not include "installing before validation"
+            The stderr should include "retry 24 5 25 nvidia-modprobe -u -c0"
+            The stderr should include "nvidia-smi"
+        End
+
+        Parameters
+            UBUNTU false amd64
+            UBUNTU "" amd64
+            UBUNTU true arm64
+            AZURELINUX true amd64
+        End
+
+        It "preserves legacy validation for OS=$1 VHD=$2 arch=$3"
+            OS=$1
+            IS_VHD=$2
+            MOCK_CPU_ARCH=$3
+            When run validateGPUDrivers
+            The status should be success
+            The output should include "gpu driver loaded"
+            The output should not include "normal driver installation"
+            The stderr should include "retry 24 5 25 nvidia-modprobe -u -c0"
+            The stderr should include "nvidia-smi"
+        End
+    End
+
     Describe 'logGPUDriverPrebakeReadiness'
         It 'reports marker_present=false when no prebake marker exists'
             GPU_DKMS_MARKER_FILE="$(mktemp)"; rm -f "${GPU_DKMS_MARKER_FILE}"

vhdbuilder/packer/README.md

@@ -28,6 +28,12 @@ Goal1: remove mariner workflow so things will be simplified.
 - `cse_config_network.sh` -> `provision_configs_network.sh`
 - `cse_config_addons.sh` -> `provision_configs_addons.sh` (autoscaler, ACI connector, Azure Policy)
 
+On managed Ubuntu amd64 VHD nodes, GPU validation selects the normal installer
+immediately if `nvidia-modprobe` is absent. A cached installer image is not an
+installed host driver. Present drivers retain the existing readiness retries and
+full-install fallback; ARM64, other OSes and non-VHD images retain their existing
+validation path. GPU eligibility and opt-out decisions still happen in `nodePrep`.
+
 `cse_cmd.sh` and the ANC parser provide their paths through
 `CSE_CONFIG_GPU_FILEPATH`, `CSE_CONFIG_LOCALDNS_FILEPATH`,
 `CSE_CONFIG_KUBELET_FILEPATH`, `CSE_CONFIG_NETWORK_FILEPATH`, and