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

["Luck, Tony" <tony.luck@intel.com>]

On Fri, Aug 21, 2026 at 02:00:41PM -0700, Reinette Chatre wrote:
> Hi Tony,
> 
> On 8/21/26 12:44 PM, Luck, Tony wrote:
> > 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@intel.com>
> >>> ---
> >>>  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).
> 
> Naming is as always complicated and here I do not see why the name should be
> picked from Intel SDM instead of AMD's spec. Renaming may be secondary goal. It may even
> add to confusion to have feature name mismatch what is exposed to user space? A change
> like this could perhaps be punted to when/if monitoring of a new resource needs to be
> supported?

Two reasons to pick the Intel SDM name:
1) Intel was here first
   commit cbc82b172638 ("x86: Add support for Intel Cache QoS Monitoring (CQM) detection")
2) Symmetry with the feature bit for allocation:
   #define X86_FEATURE_RDT_A            ( 9*32+15) /* "rdt_a" Resource Director Technology Allocation */

Confusion between the string in /proc/cpuinfo and the X86_FEATURE define
is already rampant. Just a few examples:

#define X86_FEATURE_XMM                 ( 0*32+25) /* "sse" */
#define X86_FEATURE_XSTORE              ( 5*32+ 2) /* "rng" RNG present (xstore) */
#define X86_FEATURE_SVML                (15*32+ 2) /* "svm_lock" SVM locking MSR */
#define X86_FEATURE_TSCRATEMSR          (15*32+ 4) /* "tsc_scale" TSC scaling support */

So anyone wanting to find the #define that goes with a feature name in
/proc/cpuinfo would be advised to just grep for the string in
<asm/cpufeatures.h>

> 
> > 
> > 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"
> 
> Even if the spec does not explicitly state this, the implementation requires this.
> 
> I believe patch 2/4 makes this clear since it demonstrates how resctrl obtains the general
> L3 monitoring properties from CPUID(0xF, 0x1) before moving to the ABMC feature specific
> properties.

Agreed.

> How these general L3 monitoring properties are required by ABMC can also be seen
> in the ABMC counter reading code:  resctrl_arch_cntr_read() calls get_corrected_val()
> that uses the general L3 monitoring properties hw_res->mbm_width and hw_res->mon_scale.
> ABMC feature properties also do not expose its own number of RMID supported, this is
> learned from general L3 monitoring properties.
> 
> One item of note here is just that ABMC (as I understand) does _not_ depend on any of
> the individual L3 monitoring features (X86_FEATURE_CQM_OCCUP_LLC, X86_FEATURE_CQM_MBM_TOTAL,
> and X86_FEATURE_CQM_MBM_LOCAL) since it defines its own transactions that can be counted.
> 
> Reinette

-Tony