Re: [PATCH for-4.21] x86/cpu: populate CPUID 0x1.edx features early for self-snoop detection

[Jan Beulich <jbeulich@xxxxxxxx>]

On 24.09.2025 15:40, Roger Pau Monné wrote:
> On Wed, Sep 24, 2025 at 11:50:02AM +0000, Andrew Cooper wrote:
>> On 24/09/2025 4:00 am, Roger Pau Monne wrote:
>>> Otherwise the check for the SS feature in
>>> check_memory_type_self_snoop_errata() fails unconditionally, which leads to
>>> X86_FEATURE_XEN_SELFSNOOP never being set.
>>>
>>> We could also avoid this by not doing the reset_cpuinfo() for the BSP in
>>> identify_cpu(), because SS detection uses boot_cpu_data.
>>
>> Doesn't this, mean ...
> 
> Well, that's the reason for the rant here.  The reset at the top of
> identify_cpu() has been there since 2005.  It's arguably to make sure
> the BSP and the APs have the same empty state in the passed
> cpuinfo_x86 struct, as for the BSP this would be already partially
> initialized due to what's done in early_cpu_init().
> 
> The underlying question is whether we would rather prefer to not do
> the reset for the BSP, but that would lead to differences in the
> contents of cpuinfo_x86 struct between the BSP and the APs.  In the
> past we have arranged for leaves needed early to be populated in
> generic_identify(), like FEATURESET_e21a, hence the proposed patch
> does that for FEATURESET_1d.
> 
>>>   However that
>>> creates an imbalance on the state of the BSP versus the APs in the
>>> identify_cpu() code.
>>>
>>> I've opted for the less controversial solution of populating FEATURESET_1d
>>> in generic_identify(), as the value is already there.  The same is done for
>>> the AMD faulting probe code.
>>>
>>> Fixes: f2663ca2e520 ("x86/cpu/intel: Clear cache self-snoop capability in 
>>> CPUs with known errata")
>>> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>
>> ... this Fixes tag is incorrect?
> 
> I think the Fixes tag is accurate; the code was OK before that change.
> Nothing in c_early_init hooks depended on (some of) the x86_capability
> fields being populated, which is required after the change.

I agree. Hence:
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

I wonder though whether while there we wouldn't want to also store ecx if
we already have it. (Really there is the question of whether we haven't
other cpu_has_* uses which similarly come "too early".)

Jan