Re: [PATCH for 4.19?] x86/Intel: unlock CPUID earlier for the BSP

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 13/06/2024 9:19 am, Jan Beulich wrote:
> Intel CPUs have a MSR bit to limit CPUID enumeration to leaf two. If
> this bit is set by the BIOS then CPUID evaluation does not work when
> data from any leaf greater than two is needed; early_cpu_init() in
> particular wants to collect leaf 7 data.
>
> Cure this by unlocking CPUID right before evaluating anything which
> depends on the maximum CPUID leaf being greater than two.
>
> Inspired by (and description cloned from) Linux commit 0c2f6d04619e
> ("x86/topology/intel: Unlock CPUID before evaluating anything").
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> While I couldn't spot anything, it kind of feels as if I'm overlooking
> further places where we might be inspecting in particular leaf 7 yet
> earlier.
>
> No Fixes: tag(s), as imo it would be too many that would want
> enumerating.

I also saw that go by, but concluded that Xen doesn't need it, hence why
I left it alone.

The truth is that only the BSP needs it.  APs sort it out in the
trampoline via trampoline_misc_enable_off, because they need to clear
XD_DISABLE in prior to enabling paging, so we should be taking it out of
early_init_intel().

But, we don't have an early BSP-only early hook, and I'm not overwhelmed
at the idea of exporting it from intel.c

I was intending to leave it alone until I can burn this whole
infrastructure to the ground and make it work nicely with policies, but
that's not a job for this point in the release...

~Andrew