Hacker News Reader

Nothing like that happens. Writes to dram or pcie devices or data bus cannot be rolled back. You have a misunderstanding of what speculative execution entails. The things that get rolled back are writes to general purpose registers.

How is this implemented? Undoing register renaming?

Exactly. Also not committing writes that are almost in the store buffer.

I/O takes seven eternities in microcode-ville. It doesn't hurt too much to wait for the speculation to be resolved before changing state visible outside the microarchitecture of the CPU.

Accesses to memory that is configured for device I/O (UnCacheable in x86 terms) are never speculated, so there’s nothing to roll back.