Re: [PATCH v6 00/10] Retpoline: Avoid speculative indirect calls in kernel

[Andrew Cooper]

On 08/01/18 10:42, Paul Turner wrote:
> A sequence for efficiently refilling the RSB is:
>     mov $8, %rax;
>     .align 16;
>    3: call 4f;
>   3p: pause; call 3p;
>      .align 16;
>   4: call 5f;
>   4p: pause; call 4p;
>      .align 16;
>    5: dec %rax;
>       jnz 3b;
>       add $(16*8), %rsp;
> This implementation uses 8 loops, with 2 calls per iteration.  This is
> marginally faster than a single call per iteration.  We did not
> observe useful benefit (particularly relative to text size) from
> further unrolling.  This may also be usefully split into smaller (e.g.
> 4 or 8 call)  segments where we can usefully pipeline/intermix with
> other operations.  It includes retpoline type traps so that if an
> entry is consumed, it cannot lead to controlled speculation.  On my
> test system it took ~43 cycles on average.  Note that non-zero
> displacement calls should be used as these may be optimized to not
> interact with the RSB due to their use in fetching RIP for 32-bit
> relocations.

Guidance from both Intel and AMD still states that 32 calls are required
in general. Is your above code optimised for a specific processor which
you know the RSB to be smaller on?

~Andrew