"Speculative instruction fetch to device memory crashes device" is in my list of favorite bugs ever found. I had ran into it on a Cortex-M4 without an MPU, so I couldn't even fix the memory attributes, the only solution was to gerrymander the compiled code to prevent the speculative fetch.

It's crazy that Armv8/9-A permits speculative instruction fetches to device memory. Crazy enough that I had to look it up to believe it:

"Hardware does not prevent speculative instruction fetches from a memory location with any of the Device memory attributes unless the memory location is also marked as execute-never for all Exception levels." - ARM DDI 0487K.a § B2.15.2

The Armv7-M spec is less clear. It does say: "The architecture does not permit speculative accesses to memory marked as Device," in contrast to Armv8/9-A which qualifies a similar statement with "data accesses". But then it later says "To ensure correctness, read-sensitive locations must be marked as non-executable". (This is all from ARM DDI 0403E.e § A3.5.7)

I don't have v7-A older handy to compare what they say.