Re: [PATCH v2 1/5] perf/x86/intel/lbr: use setup_clear_cpu_cap instead of clear_cpu_cap

Maxim Levitsky
2022-09-20T08:23:24Z

On Mon, 2022-09-19 at 16:31 +0200, Borislav Petkov wrote:
> On Mon, Jul 18, 2022 at 05:11:19PM +0300, Maxim Levitsky wrote:
> > clear_cpu_cap(&boot_cpu_data) is very similar to setup_clear_cpu_cap
> > except that the latter also sets a bit in 'cpu_caps_cleared' which
> > later clears the same cap in secondary cpus, which is likely
> > what is meant here.
> >
> > Fixes: 47125db27e47 ("perf/x86/intel/lbr: Support Architectural LBR")
> >
> > Reviewed-by: Kan Liang <kan.liang@linux.intel.com>
> > Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
> > ---
> >  arch/x86/events/intel/lbr.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/events/intel/lbr.c b/arch/x86/events/intel/lbr.c
> > index 13179f31fe10fa..b08715172309a7 100644
> > --- a/arch/x86/events/intel/lbr.c
> > +++ b/arch/x86/events/intel/lbr.c
> > @@ -1860,7 +1860,7 @@ void __init intel_pmu_arch_lbr_init(void)
> >         return;
> >
> >  clear_arch_lbr:
> > -       clear_cpu_cap(&boot_cpu_data, X86_FEATURE_ARCH_LBR);
> > +       setup_clear_cpu_cap(X86_FEATURE_ARCH_LBR);
>
> setup_clear_cpu_cap() has a very specific purpose - see
> apply_forced_caps().
>
> This whole call sequence is an early_initcall() which is way after the
> whole CPU features picking apart happens.
>
> So what is actually this fixing?
>

If I understand that correctly, the difference between clear_cpu_cap and setup_clear_cpu_cap
is that setup_clear_cpu_cap should be called early when only the boot cpu is running and it

1. works on 'boot_cpu_data' which represents the boot cpu.
2. sets a bit in 'cpu_caps_cleared' which are later applied to all CPUs, including these that are hotplugged.

On the other hand the clear_cpu_cap just affects the given 'struct cpuinfo_x86'.

Call of 'clear_cpu_cap(&boot_cpu_data, X86_FEATURE_ARCH_LBR)' is weird since it still affects 'boot_cpu_data'
but doesn't affect 'cpu_caps_cleared'

I assumed that this was a mistake and the intention was to disable the feature on all CPUs.

I need this patch because in the next patch, I change the clear_cpu_cap such as it detects being
called on boot_cpu_data and in this case also clears bits in 'cpu_caps_cleared', thus
while this patch does introduce a functional change, the next patch doesn't since this is the only
place where clear_cpu_cap is called explicitly on 'boot_cpu_data'

I do now notice that initcalls are run after smp is initialized, which means that this code doesn't really
disable the CPUID feature on all CPUs at all.

Maybe we can drop the call instead, which does seem to be wrong?

Best regards,
Maxim Levitsky