Re: [PATCH 3/7] x86/enter: Use IBRS on syscall and interrupts

[Dave Hansen]

On 01/04/2018 08:51 PM, Andy Lutomirski wrote:
> Do we need an arch_prctl() to enable IBRS for user mode?

Eventually, once the dust settles.  I think there's a spectrum of
paranoia here, that is roughly (with increasing paranoia):

1. do nothing
2. do retpoline
3. do IBRS in kernel
4. do IBRS always

I think you're asking for ~3.5.

Patches for 1-3 are out there and 4 is pretty straightforward.  Doing a
arch_prctl() is still straightforward, but will be a much more niche
thing than any of the other choices.  Plus, with a user interface, we
have to argue over the ABI for at least a month or two. ;)