Re: [PATCH for-4.7/4.8] x86: Fix "x86: further CPUID handling adjustments"

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 16/05/18 09:14, Jan Beulich wrote:
>>>> On 15.05.18 at 19:54, <andrew.cooper3@citrix.com> wrote:
>> Also, I don't see any link between the change and the commit message.  With
>> the microcode installed, STIBP and IBPB are already visible to dom0.
> They reportedly weren't (and I was able to confirm that), and given this
> original (prior to that change) code
>
>             }
>         }
>         else
>             b = c = 0;
>         a = d = 0;
>
> I also can't see how IBRSB and STIBP could have been visible. I agree I
> had wrongly extended that to IBPB.
>
>> The only required adjustment is to force STIBP == IBRSB, which must be done
>> after applying the pv_featureset[] mask to the toolstack's choice of value.
> I can see how I've got that part wrong from a leveling perspective (I was
> really too focused on Dom0 back then), but I don't see how reporting IBPB
> when IBRSB is available in hardware (implying IBPB itself isn't) would work
> with your change in place.

I've submitted v2 with an updated commit message, now I understand where
the dom0 comment came from.

>
> I'm also not convinced assimilating Sergey's original change into this one is
> appropriate - raw_featureset[] isn't used for anything except the sysctl.

You regressed a feature with an incorrect backport, in a way which
directly impacts a tool which administrators will use to see if they've
got the microcode applied properly.

It was broken in a security patch, therefore it is going to get fixed.

~Andrew

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