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

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