Re: [RFC PATCH 31/56] x86/alternative: Prepend nops with retpolines

Peter Zijlstra
2025-10-16T10:32:46Z

On Mon, Oct 13, 2025 at 09:34:19AM -0500, David Kaplan wrote:
> When patching retpolines, nops may be required for padding such as when
> turning a 5-byte direct call into a 2-byte indirect call. Previously,
> these were appended at the end so the code becomes "call *reg;nop;nop;nop"
> for example. This was fine because it's always going from a larger
> instruction to a smaller one.
>
> But this is a problem if the sequence is transformed from a 2-byte indirect
> to the 5-byte direct call version at runtime because when the called
> function returns, it will be in the middle of the 5-byte call instruction.
>
> To fix this, prepend the nops instead of appending them. Consequently, the
> return site of the called function is always the same.
>
> For indirect jmps this is potentially slightly less efficient compared to
> appending nops, but indirect jmps are so rare this hardly seems worth
> optimizing.

Durr..

So Retpoline sites can be 5 or 6 bytes, depending on the register.

Also, I suppose at this point I would prefer prefix stuffing
over multiple instructions. The prefix stuffing ensures it stays a
single instruction.

Alas, some micro-archs have significant decode penalties for >3
prefixes, and filling 6 bytes will need 4 prefixes :-(