* Nadav Amit <nadav.a...@gmail.com> wrote:
> > So we are trading a 5-15% slowdown (PTI) for another 5-15% slowdown, plus
> > we
> > are losing the soft-SMEP feature on older CPUs that PTI enables, which is a
> > pretty powerful mitigation technique.
>
> This soft-SMEP can be kept by keeping PTI if SMEP is unsupported. Although we
> trade slowdowns, they are different ones, which allows the user to make his
> best
> decision.
Indeed, not allowing PTI to be disabled if SMEP is unavailable might be a
solution.
> > Yes, I suspect in some (maybe many) cases it would be a speedup, but I
> > really
> > don't like the underlying assumptions and tradeoffs here. (Not that I like
> > any
> > of this whole Meltdown debacle TBH.)
>
> To make sure that I understand correctly - the assumptions are that disabling
> PTI on compatibility mode would: (1) Benefit some workloads; (2) Be useful,
> even
> if we only consider CPUs with SMEP; and (3) Secure.
>
> Under these assumptions, the tradeoff is slightly greater code complexity for
> considerably better performance of 32-bit code; in some common cases this
> makes
> 32-bit code to perform significantly better than 64-bit code.
>
> Am I missing something? My main concern was initially security, but so far
> from
> your aggregated feedback I did not see something concrete which cannot
> relatively easily be addressed.
Yes, I suppose.
Thanks,
Ingo
