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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Sep 25, 2025 at 09:41:43AM +0200, Jan Beulich wrote:
> 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?

I always err on the side of caution and ask for them.  Maybe Oleksii
can state if/when he formally wants release-acks for bugfixes.

Regards, Roger.