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

["Roger Pau Monné" <roger.pau@citrix.com>]

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@citrix.com>
> >>>>
> >>>> ... 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@suse.com>
> >>
> >> 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.

Thanks!

Oleksii, can I please get a release-ack for this to go in?

Regards, Roger.