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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 13.06.2024 18:17, Andrew Cooper wrote:
> 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().

Except for the (odd) case also mentioned to Roger, where the BSP might have
the bit clear but some (or all) AP(s) have it set.

> 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...

This last part reads like the rest of your reply isn't an objection to me
putting this in with Roger's R-b, but it would be nice if you could
confirm this understanding of mine. Without this last part it, especially
the 2nd from last paragraph, certainly reads a little like an objection.

Jan