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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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