[llvm-dev] retpoline mitigation and 6.0

David Woodhouse via llvm-dev llvm-dev at lists.llvm.org
Fri Feb 2 16:36:27 PST 2018

On Sat, 2018-02-03 at 00:23 +0000, Chandler Carruth wrote:
> Two aspects to this...
> One, we're somewhat reluctant to guarantee an ABI here. At least I
> am. While we don't *expect* rampant divergence here, I don't want
> this to become something we cannot change if there are good reasons
> to do so. We've already changed the thunks once based on feedback
> (putting LFENCE after the PAUSE).

Surely adding the lfence was changing your implementation, not the ABI?

And if we really are talking about the *ABI* not the implementation,
I'm not sure I understand your concern.

The ABI for each thunk is that it is identical in all respects, apart
from speculation, to 'jump *\reg'. There's not a lot of scope for a
'v2' of that, surely?

I could live with the command-line divergence, although it's
suboptimal. But *please* since these things also need to be symbols
exported to loadable modules, can't we keep the thunk names identical?

You can even use __x86_indirect_thunk_\reg for *now* and reserve the
right to use a different name if you ever do revise the ABI.

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