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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 25.09.2025 09:40, Roger Pau Monné wrote:
> On Thu, Sep 25, 2025 at 09:37:46AM +0200, Jan Beulich wrote:
>> On 25.09.2025 09:34, Roger Pau Monné wrote:
>>> On Thu, Sep 25, 2025 at 09:03:06AM +0200, Jan Beulich wrote:
>>>> 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".)
>>>
>>> Yeah, I was about to do it, but it's not strictly needed for
>>> c_early_init, and it's done anyway just after the call to
>>> c_early_init.  I can set that field also, but then I would need to
>>> tweak the comment ahead, something like:
>>
>> Sure, i.e. fine with me.
> 
> Oleksii, can I please get a release-ack for this to go in?

Do bug fixes actually need release-acks just yet?

Jan