Messages in this thread |  | | Subject | Re: [PATCH] x86/cpuid: Fix up "virtual" IBRS/IBPB/STIBP feature bits on Intel | From | David Woodhouse <> | Date | Tue, 30 Jan 2018 11:28:13 +0000 |
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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.[unhandled content-type:application/x-pkcs7-signature]
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