Re: [PATCH 2/4] perf/x86/intel: Do not deploy workaround when TSX is deprecated

[Pawan Gupta]

On 11.06.2021 09:50, Borislav Petkov wrote:
> On Wed, Jun 09, 2021 at 02:12:38PM -0700, Pawan Gupta wrote:
> > Earlier workaround added by commit 400816f60c54 ("perf/x86/intel:
> > Implement support for TSX Force Abort") for perf counter interactions
> > [1] are not required on some client systems which received a microcode
> > update that deprecates TSX.
> >
> > Bypass the perf workaround when such microcode is enumerated.
> >
> > [1] Performance Monitoring Impact of Intel® Transactional Synchronization Extension Memory
> >     http://cdrdv2.intel.com/v1/dl/getContent/604224
> >
> > Signed-off-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
> > Reviewed-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> > Reviewed-by: Tony Luck <tony.luck@xxxxxxxxx>
> > Tested-by: Neelima Krishnan <neelima.krishnan@xxxxxxxxx>
> > ---
> >  arch/x86/events/intel/core.c | 22 ++++++++++++++++++----
> >  1 file changed, 18 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
> > index e28892270c58..b5953e1e59a2 100644
> > --- a/arch/x86/events/intel/core.c
> > +++ b/arch/x86/events/intel/core.c
> > @@ -6016,10 +6016,24 @@ __init int intel_pmu_init(void)
> >  		intel_pmu_pebs_data_source_skl(pmem);
> >
> >  		if (boot_cpu_has(X86_FEATURE_TSX_FORCE_ABORT)) {
> > -			x86_pmu.flags |= PMU_FL_TFA;
> > -			x86_pmu.get_event_constraints = tfa_get_event_constraints;
> > -			x86_pmu.enable_all = intel_tfa_pmu_enable_all;
> > -			x86_pmu.commit_scheduling = intel_tfa_commit_scheduling;
> > +			u64 msr;
> > +
> > +			rdmsrl(MSR_TSX_FORCE_ABORT, msr);
> > +			/* Systems that enumerate CPUID.RTM_ALWAYS_ABORT or
> > +			 * support MSR_TSX_FORCE_ABORT[SDV_ENABLE_RTM] bit have
> > +			 * TSX deprecated by default. TSX force abort hooks are
> > +			 * not required on these systems.
>
> So if they're not required, why aren't you simply disabling the force
> abort "workaround" by clearing the feature flag?

Feature flag also enumerates MSR_TSX_FORCE_ABORT, which is still present
after the microcode update. Patch 3/4 in this series clears the TSX
CPUID bits using MSR_TSX_FORCE_ABORT. So we do need the feature flag 
X86_FEATURE_TSX_FORCE_ABORT.

> 	if (boot_cpu_has(X86_FEATURE_TSX_FORCE_ABORT)) {
> 		if (boot_cpu_has(X86_FEATURE_RTM_ALWAYS_ABORT))
> 			setup_clear_cpu_cap(X86_FEATURE_TSX_FORCE_ABORT);
> 	}
>
> so that it doesn't get enabled in the first place?

Feature flag is needed as explained above.

Thanks,
Pawan