Re: [PATCH v11 01/23] x86/resctrl: Give better names to X86_FEATURE flags for monitoring

[Luck, Tony]

On Wed, Sep 09, 2026 at 08:48:01PM -0700, Reinette Chatre wrote:
> Hi Tony,
> 
> Please switch the subject prefix to "x86/cpufeatures:" to highlight the
> subsystem changed. Considering that, the short description could mention
> resctrl instead, for example:
> 	x86/cpufeatures: Give better names to flags used by resctrl
> 
> Although, I do have a question about one rename and depending on that
> outcome the subject could be made more specific.
> 
> On 8/31/26 10:43 AM, Tony Luck wrote:
> > The feature flags for enumeration of Resource Director Technology (RDT)
> > capabilities were chosen when the only feature was LLC cache occupancy
> > monitoring and they were given names using the abbreviation CQM for
> > Cache Quality of Service Monitoring.
> > 
> > Additional monitoring features have been added to CPUs and the names
> > are now more likely to confuse than inform the purpose of these flags.
> > 
> > Rename X86_FEATURE_CQM to X86_FEATURE_RDT_M (to match the Intel Software
> > Developer's Manual, and for symmetry with X86_FEATURE_RDT_A).
> > 
> > Rename X86_FEATURE_CQM_LLC to X86_FEATURE_L3_MON since it enumerates
> > that some L3 monitoring features may be present.
> 
> Considering LLC as synonym for L3 it is not obvious why this rename is needed
> (more below). 

I'm concerned about the "CQM" string in the name. This stands for "Cache
QoS Monitoring" according to the original commit that added it:

 cbc82b172638 ("x86: Add support for Intel Cache QoS Monitoring (CQM) detection")

This made perfect sense when the only feature was LLC cache occupancy.
That's very clearly a useful metric for anyone interested in cache
quality of service.

But the next two monitoring features added to CPUID leaf 0xF subleaf 0x1
were memory bandwidth monitoring of local & total traffic. These are
only peripherally connected to cache quality of service.

I realize that this argument is somewhat undercut by the Linux naming of
those features with CQM substrings:

#define X86_FEATURE_CQM_MBM_TOTAL       (11*32+ 2) /* "cqm_mbm_total" LLC Total MBM monitoring */
#define X86_FEATURE_CQM_MBM_LOCAL       (11*32+ 3) /* "cqm_mbm_local" LLC Local MBM monitoring */

But those seem wrong too.

> > 
> > Add missing dependency to cpuid_deps[].
> 
> nit: dependency -> dependencies
> 
> > diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> > index f70ee74b5f92..6a7f0adb123e 100644
> > --- a/arch/x86/include/asm/cpufeatures.h
> > +++ b/arch/x86/include/asm/cpufeatures.h
> 
> ...
> 
> > @@ -285,7 +285,7 @@
> >   *
> >   * Reuse free bits when adding new feature flags!
> >   */
> > -#define X86_FEATURE_CQM_LLC		(11*32+ 0) /* "cqm_llc" LLC QoS if 1 */
> > +#define X86_FEATURE_L3_MON		(11*32+ 0) /* "cqm_llc" LLC QoS if 1 */
> 
> The original name matched the description and since the description needed no changing it
> is not clear why the feature name needed to change? I do see some redundancy in the name with
> "LLC" as well "cache" making an appearance, but none of that is inaccurate, is it? The changelog
> claims that the name confuses the purpose. How does X86_FEATURE_CQM_LLC confuse the purpose of
> the flag?

See above ... 
> 
> >  #define X86_FEATURE_CQM_OCCUP_LLC	(11*32+ 1) /* "cqm_occup_llc" LLC occupancy monitoring */
> >  #define X86_FEATURE_CQM_MBM_TOTAL	(11*32+ 2) /* "cqm_mbm_total" LLC Total MBM monitoring */
> >  #define X86_FEATURE_CQM_MBM_LOCAL	(11*32+ 3) /* "cqm_mbm_local" LLC Local MBM monitoring */
> Reinette
> 

-Tony