doc: clarify cross-kernel snapshot compatibility

Simplify by just making the statement that we do not recommend it, and
it generally doesnt work except for one limited case. Simplify the table
by only listing what's working, an by having the first column directly
refer to instance types (instead of cpu architectures, which we later
map to instance types in the text).

Closes firecracker-microvm#4912
Signed-off-by: Patrick Roy <[email protected]>

Author: roypat

docs/snapshotting/snapshot-support.md

@@ -640,26 +640,17 @@ booting, to avoid this issue.
 
 ## Snapshot compatibility across kernel versions
 
-We have a mechanism in place to experiment with snapshot compatibility across
-supported host kernel versions by generating snapshot artifacts through
-[this test](../../tests/integration_tests/functional/test_snapshot_phase1.py)
-and checking devices' functionality using
-[this test](../../tests/integration_tests/functional/test_snapshot_restore_cross_kernel.py).
-The test restores the snapshot and ensures that all the devices set-up (network
-devices, disk, vsock, balloon and MMDS) are operational post-load.
-
-In those tests the instance is fixed, except some combinations where we also
-test across the same CPU family (Intel x86, Gravitons). In general cross-CPU
-snapshots [are not supported](./versioning.md#cpu-model)
-
-The tables below reflect the snapshot compatibility observed on the AWS
-instances we support.
-
-**all** means all currently supported Intel/AMD/ARM metal instances (m6g, m7g,
-m5n, c5n, m6i, m6a). It does not mean cross-instance, i.e. a snapshot taken on
-m6i won't work on an m6g instance.
-
-| *CPU family* | *taken on host kernel* | *restored on host kernel* | *working?* |
-| ------------ | ---------------------- | ------------------------- | ---------- |
-| **x86_64**   | 5.10                   | 6.1                       | Y          |
-| **x86_64**   | 6.1                    | 5.10                      | N          |
+Snapshots must be resumed on an software and hardware configuration which is
+identical to what they were generated on. However, in limited cases, snapshots
+can be resumed on identical hardware instances where they were taken on, but
+using newer host kernel versions. While we do not provide any guarantees on this
+setup (and do not recommend doing this in production), we are currently aware of
+the compatibility table reported below:
+
+| .metal instance type | taken on host kernel | restored on host kernel |
+| -------------------- | -------------------- | ----------------------- |
+| {c5n,m5n,m6i,m6a}    | 5.10                 | 6.1                     |
+
+For example, a snapshot taken on a m6i.metal host running a 5.10 host kernel can
+be restored on a different m6i.metal host running a 6.1 host kernel (but not
+vice versa), but could not be restored on a c5n.metal host.