Re: [dpdk-dev] [RFC UNTESTED PATCH] eal_common_cpuflags: Fix %rbx corruption, and simplify the code

[Neil Horman <nhorman@tuxdriver.com>]

On Thu, Mar 20, 2014 at 10:03:53AM -0700, H. Peter Anvin wrote:
> I just realized there is yet another oddity in this code:
> 
> > @@ -78,8 +69,10 @@ struct cpuid_parameters_t {
> >  struct feature_entry {
> >  	enum rte_cpu_flag_t feature;            /**< feature name */
> 
> The structure contains a field with an enum value...
> 
> >  	char name[CPU_FLAG_NAME_MAX_LEN];       /**< String for printing */
> > -	struct cpuid_parameters_t params;       /**< cpuid parameters */
> > -	uint32_t feature_mask;                  /**< bitmask for feature */
> > +	uint32_t leaf;				/**< cpuid leaf */
> > +	uint32_t subleaf;			/**< cpuid subleaf */
> > +	uint32_t reg;				/**< cpuid register */
> > +	uint32_t bit;				/**< cpuid register bit */
> >  };
> >  
> >  
> >  /*
> > @@ -240,17 +207,20 @@ rte_cpu_get_features(struct cpuid_parameters_t params)
> >  int
> >  rte_cpu_get_flag_enabled(enum rte_cpu_flag_t feature)
> >  {
> > -	int value;
> > +	const struct feature_entry *feat;
> > +	cpu_registers_t regs;
> >  
> >  	if (feature >= RTE_CPUFLAG_NUMFLAGS)
> >  		/* Flag does not match anything in the feature tables */
> >  		return -ENOENT;
> >  
> > -	/* get value of the register containing the desired feature */
> > -	value = rte_cpu_get_features(cpu_feature_table[feature].params);
> > +	feat = &cpu_feature_table[feature];
> > +
> > +	/* get the cpuid leaf containing the desired feature */
> > +	rte_cpu_get_features(feat->leaf, feat->subleaf, &regs);
> >  
> >  	/* check if the feature is enabled */
> > -	return (cpu_feature_table[feature].feature_mask & value) > 0;
> > +	return (regs[feat->reg] >> feat->bit) & 1;
> >  }
> >  
> >  /**
> 
> ... however, this field is never actually accessed *anywhere* in the
> code; the code instead uses the enum value as the table index. There is
> absolutely no enforcement that the table contents is aligned with the enum.
> 
> If C99-style initializers are permitted in this codebase, I would
> strongly recommend using them, and then drop the enum field in struct
> feature_entry and use a macro such as:
> 
Actually, its a bit simpler than that, the enum parameter is actually completely
unused, and so can be removed entirely.  The FEAT_DEF macro does what you
suggest below already, but only for the feature and name fields.

I'll remove the enum and its definition, and augment the macro to cover the rest
of the fields.

Neil

> #define FEAT(name,leaf,subleaf,reg,bit) \
> 	[RTE_CPUFLAG_##f] = { leaf, subleaf, reg, bit, #f },
> 
> (I'd move the string to the end, but that is just a microoptimization.
> I'm kind of OCD that way.)
> 
> 	-hpa
> 
>