Re: [PATCH] x86/cpuid: Fix up "virtual" IBRS/IBPB/STIBP feature bits on Intel

[David Woodhouse]

On Tue, 2018-01-30 at 12:18 +0100, Borislav Petkov wrote:
> On Tue, Jan 30, 2018 at 11:03:50AM +0000, David Woodhouse wrote:
> > 
> > I pondered that, but I didn't like it. I didn't want to always *force*
> > those features on, for all CPUs, just because they happened to be
> > discovered at boot time on the first CPU (which *did* have its
> > microcode updated by the crappy BIOS, while the others didn't).
> > 
> > I strongly suspect that's purely an academic concern, and we mostly
> > check boot_cpu_has() and never even *notice* if secondary CPUs don't
> > match. I just didn't want to make that *worse*. It tickled my OCD.
>
> Well, you need to do it because those bits are AMD-specific and they are
> not set in the Intel CPUID leaf and identify_cpu() towards the end takes
> care of "ironing" all those bits out which are not part of the common
> feature set and which get_cpu_cap() has *not* read out from CPUID.

I need to set them for each CPU which has the Intel hardware bits set,
sure. I don't need to use setup_force_cpu_cap() to do it. The patch I
sent was doing it for each CPU.

> It is one of those I-told-you-so moments when I suggested to make the
> visible feature bits the artificial ones and have the *actual* hardware
> ones set those.

We don't have artificial ones for the hardware capability, but yes I
could add another three. I could add X86_FEATURE_IBRS which is a
virtual bit, set when *either* X86_FEATURE_SPEC_CTRL (on Intel) or
X86_FEATURE_AMD_IBRS (on AMD) is set.

But actually... that doesn't help, does it? Because early_init_intel()
is still only called *once* for the boot CPU. Those software bits would
be set... and perhaps not later cleared when identify_boot_cpu()
happens later, but would they ever get set for secondary CPUs? The code
to set those virtual bits would *still* need to live somewhere that
will get called for secondary CPUs, as I've done in this patch.

I could use setup_force_cpu_cap() but I still don't like that, as
discussed.

So no, I don't see why inventing three more "virtual" bits to precisely
parallel the AMD bits would really make much difference.

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