feat(test): add linux 6.1 to 6.18 cross-kernel snapshot restore testing

Add AL2023/linux_6.18 as a restore-only platform in the cross-snapshot
pipeline for both x86_64 and aarch64. Snapshots created on 6.1 hosts are
restored on 6.18 hosts to validate cross-kernel compatibility. The 6.18
platform is scoped to pipeline_cross.py only since 6.18 agents exist
exclusively in the private Buildkite queue.

Signed-off-by: Jack Thomson <jackabt@amazon.com>

Author: JackThomson2

.buildkite/pipeline_cross.py

@@ -27,6 +27,8 @@
         "m7a.metal-48xl",
     ]
     instances_aarch64 = ["m7g.metal"]
+    restore_only_platforms = [("al2023", "linux_6.18")]
+    x86_64_platforms = DEFAULT_PLATFORMS + restore_only_platforms
     commands = [
         "./tools/devtool -y test --no-build --no-archive -- -m nonci -n4 integration_tests/functional/test_snapshot_phase1.py",
         # punch holes in mem snapshot tiles and tar them so they are preserved in S3
@@ -54,12 +56,13 @@
 
     # https://github.com/firecracker-microvm/firecracker/blob/main/docs/kernel-policy.md#experimental-snapshot-compatibility-across-kernel-versions
     aarch64_platforms = [("al2023", "linux_6.1")]
+    aarch64_all_platforms = aarch64_platforms + restore_only_platforms
     perms_aarch64 = itertools.product(
-        instances_aarch64, aarch64_platforms, instances_aarch64, aarch64_platforms
+        instances_aarch64, aarch64_platforms, instances_aarch64, aarch64_all_platforms
     )
 
     perms_x86_64 = itertools.product(
-        instances_x86_64, DEFAULT_PLATFORMS, instances_x86_64, DEFAULT_PLATFORMS
+        instances_x86_64, DEFAULT_PLATFORMS, instances_x86_64, x86_64_platforms
     )
     steps = []
     for (
@@ -74,6 +77,9 @@
         # newer -> older is not supported, and does not work
         if src_kv > dst_kv:
             continue
+        # only test cross-kernel restore between adjacent kernel versions
+        if src_kv == "linux_5.10" and dst_kv == "linux_6.18":
+            continue
         if src_instance != dst_instance and dst_instance not in supported.get(
             src_instance, []
         ):