Re: [RFC PATCH 1/4] x86/resctrl: Check if monitoring features are enabled

[Luck, Tony]

On Fri, Aug 21, 2026 at 11:18:51AM -0700, Reinette Chatre wrote:
> Hi Tony,
> 
> Thank you for doing this.
> 
> On 8/19/26 9:13 AM, Tony Luck wrote:
> > Both Intel and AMD manuals say that software must first check
> > CPUID(0x7,0x0).EBX[12] to see if any monitoring features are enabled
> > before checking for specific features enabled in subleaves.
> > 
> > Add the check for X86_FEATURE_CQM.
> > 
> > Fixes: cbc82b172638 ("x86: Add support for Intel Cache QoS Monitoring (CQM) detection")
> > Signed-off-by: Tony Luck <tony.luck@xxxxxxxxx>
> > ---
> >  arch/x86/kernel/cpu/resctrl/core.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/arch/x86/kernel/cpu/resctrl/core.c b/arch/x86/kernel/cpu/resctrl/core.c
> > index 55214d6fdc49..2677b8a6c15b 100644
> > --- a/arch/x86/kernel/cpu/resctrl/core.c
> > +++ b/arch/x86/kernel/cpu/resctrl/core.c
> > @@ -968,6 +968,9 @@ static __init bool get_rdt_mon_resources(void)
> >  	struct rdt_resource *r = &rdt_resources_all[RDT_RESOURCE_L3].r_resctrl;
> >  	bool ret = false;
> >  
> > +	if (!cpu_feature_enabled(X86_FEATURE_CQM))
> > +		return false;
> 
> Is this missing a check of X86_FEATURE_CQM_LLC also?
> 
> As I understand resctrl learns from X86_FEATURE_CQM whether the system supports
> resource monitoring in general. Contrary to what the feature name suggests, there is
> another step needed to determine which resource(s) support monitoring via
> CPUID(0xF,0x0).EDX where bit 1 indicates LLC monitoring that needs to be set before
> the LLC resource-specific monitoring properties can/should be determined.

Yes. Missing that check. I agree that the feature define names are bad.

To avoid continued confusion I should rename the #defines to match the
bit names in the Intel SDM (but leaving the /proc/cpuinfo visible
strings at "cqm" and "cqm_llc" as those are user ABI now).

X86_FEATURE_CQM -> X86_FEATURE_RDT_M
X86_FEATURE_CQM_LLC -> X86_FEATURE_L3_MON

> 
> Apart from the checks here I see that cpuid_deps[] accurately reflects the relationship
> between X86_FEATURE_CQM_LLC and the different LLC monitoring features checked for below.
> I do not see cpuid_deps[] capturing the relationship between X86_FEATURE_CQM and
> X86_FEATURE_CQM_LLC though. Could adding it complete the handling of relationships between
> these leaves?

Yes.
> 
> Similarly I think cpuid_deps[] may be missing X86_FEATURE_ABMC's dependency on 
> X86_FEATURE_CQM_LLC. 

Maybe no? X86_FEATURE_ABMC isn't enumerated in CPUID(0xF,*). It comes
from the AMD CPUID(0x80000020,0)EBX{5}

Babu: The AMD architecture programmer's manual just says:

  "Support for ABMC is identified by CPUID Fn8000_0020_EBX_x0[ABMC] (bit 5)
   being set. If ABMC is supported, the feature’s attributes and capabilities
   are enumerated by CPUID Fn8000_0020_x5 as detailed in Appendix E of APM volume 3"

It isn't explicitly stated whether this depends on CPUID Fn0000_000F_x0
EDX{1}, which AMD names: "L3CacheMon - L3 monitoring capability"

> > +
> >  	if (rdt_cpu_has(X86_FEATURE_CQM_OCCUP_LLC)) {
> >  		resctrl_enable_mon_event(QOS_L3_OCCUP_EVENT_ID, false, 0, NULL);
> >  		ret = true;
> 
> Reinette

-Tony