probe: which display does a runner without a GPU accept?

Author: Letdown2491

.github/workflows/kvm-probe.yml

@@ -65,11 +65,16 @@ jobs:
 
       - name: kvm actually executes guest code
         run: |
+          set +e
           k=$(ls /boot/vmlinuz-* 2>/dev/null | head -1)
+          echo "kernel image: ${k:-none found}"
           if [ -z "$k" ]; then echo "PROBE: no kernel on runner, skipped"; exit 0; fi
+          # Runner kernels are commonly mode 600, so read it as root.
+          sudo cp "$k" /tmp/vmlinuz && sudo chmod a+r /tmp/vmlinuz
           timeout 90 qemu-system-x86_64 -accel kvm -cpu host -smp 2 -m 1024 \
               -display none -serial stdio -no-reboot \
-              -kernel "$k" -append "console=ttyS0 panic=-1" 2>&1 | head -40 | tee out
+              -kernel /tmp/vmlinuz -append "console=ttyS0 panic=-1" > out 2>&1
+          head -25 out
           grep -q 'Linux version' out \
               && echo "PROBE: guest kernel BOOTED under kvm" \
               || echo "PROBE: guest did not boot"
@@ -88,6 +93,27 @@ jobs:
           [ $rc -eq 0 ] && echo "PROBE: virtio-vga-gl + egl-headless ok" \
                         || echo "PROBE: virtio-vga-gl + egl-headless FAILED rc=$rc"
 
+      # egl-headless wants a DRM render node and a hosted runner has no GPU.
+      # smoke.sh already takes the display as $QEMU_DISPLAY, so the question
+      # is which alternative gives the guest a DRM device to run a
+      # compositor on. Software rendering in the guest is fine; what is not
+      # fine is a guest with no GPU device at all.
+      - name: display modes without a host GPU
+        run: |
+          set +e
+          ls -l /dev/dri 2>/dev/null || echo "no /dev/dri on the host"
+          sudo modprobe vgem 2>/dev/null && ls -l /dev/dri 2>/dev/null
+          for combo in "-device virtio-vga -display none" \
+                       "-device virtio-gpu-pci -display none" \
+                       "-vga std -display none" \
+                       "-device virtio-vga-gl -display egl-headless"; do
+            timeout 30 qemu-system-x86_64 -enable-kvm -cpu host -m 512 \
+                $combo -S -monitor stdio <<< 'quit' > out 2>&1
+            rc=$?
+            [ $rc -eq 0 ] && echo "PROBE: ok      $combo" \
+                          || echo "PROBE: FAILED  $combo  ($(head -1 out))"
+          done
+
       - name: loop devices and partition scanning
         run: |
           set +e