Re: [PATCH 0/1] x86/speculation: Allow IBPB to be conditionally enabled on CPUs with always-on STIBP

[Anand K. Mistry]

On Sun, 1 Nov 2020 at 01:50, Tom Lendacky <thomas.lendacky@xxxxxxx> wrote:
>
> On 10/29/20 1:51 AM, Anand K Mistry wrote:
> > When attempting to do some performance testing of IBPB on and AMD
> > platform, I noticed the IBPB instruction was never being issued, even
> > though it was conditionally on and various seccomp protected processes
> > were force enabling it. Turns out, on those AMD CPUs, STIBP is set to
> > always-on and this was causing an early-out on the prctl() which turns
> > off IB speculation. Here is my attempt to fix it.
> >
> > I'm hoping someone that understands this better than me can explain why
> > I'm wrong.
>
> It all looks reasonable to me (some comments in the patch to follow). The
> thing that makes this tough is the command line option of being able to
> force IBPB using the "prctl,ibpb" or "seccomp,ibpb" while STIBP is prctl
> or seccomp controlled. There's an inherent quality that is assumed that if
> STIBP is forced then IBPB must be forced and it looks like 21998a351512
> ("x86/speculation: Avoid force-disabling IBPB based on STIBP and enhanced
> IBRS.") used that. However, with the STIBP always on support, that doesn't
> hold true.

Yeah, and this is what I found confusing. With that commit, the number
of combinations
of IBPB and STIBP is 25, but only a small subset is possible. For example:
- (spectre_v2_user_ibpb == SPECTRE_V2_USER_STRICT &&
spectre_v2_user_stibp == SPECTRE_V2_USER_STRICT)
- (spectre_v2_user_ibpb == SPECTRE_V2_USER_STRICT &&
spectre_v2_user_stibp == SPECTRE_V2_USER_NONE)
are the only possible combinations of STRICT.

But also, if 'spectre_v2_user=seccomp,ibpb' (or prctl,ibpb), then
spectre_v2_user_ibpb == SPECTRE_V2_USER_SECCOMP even though it is
logically SPECTRE_V2_USER_STRICT.

It took a bit of time to wrap my head around this, hence I'm a bit
hesitant about this change (even though I think it's right).

>
> Thanks,
> Tom
>
> >
> >
> > Anand K Mistry (1):
> >    x86/speculation: Allow IBPB to be conditionally enabled on CPUs with
> >      always-on STIBP
> >
> >   arch/x86/kernel/cpu/bugs.c | 41 +++++++++++++++++++++-----------------
> >   1 file changed, 23 insertions(+), 18 deletions(-)
> >