Re: [dpdk-dev] [PATCH v3 01/18] eal: add max SIMD bitwidth

[Bruce Richardson <bruce.richardson@intel.com>]

On Thu, Oct 08, 2020 at 01:07:26PM +0000, Ananyev, Konstantin wrote:
> 
> > This patch adds a max SIMD bitwidth EAL configuration. The API allows
> > for an app to set this value. It can also be set using EAL argument
> > --force-max-simd-bitwidth, which will lock the value and override any
> > modifications made by the app.
> > 
> > Signed-off-by: Ciara Power <ciara.power@intel.com>
> > 
> > ---
> > v3:
> >   - Added enum value to essentially disable using max SIMD to choose
> >     paths, intended for use by ARM SVE.
> >   - Fixed parsing bitwidth argument to return an error for values
> >     greater than uint16_t.
> > v2: Added to Doxygen comment for API.
> > ---
> >  lib/librte_eal/common/eal_common_options.c | 64 ++++++++++++++++++++++
> >  lib/librte_eal/common/eal_internal_cfg.h   |  8 +++
> >  lib/librte_eal/common/eal_options.h        |  2 +
> >  lib/librte_eal/include/rte_eal.h           | 33 +++++++++++
> >  lib/librte_eal/rte_eal_version.map         |  4 ++
> >  5 files changed, 111 insertions(+)
> > 
> > diff --git a/lib/librte_eal/common/eal_common_options.c b/lib/librte_eal/common/eal_common_options.c
> > index a5426e1234..e9117a96af 100644
> > --- a/lib/librte_eal/common/eal_common_options.c
> > +++ b/lib/librte_eal/common/eal_common_options.c
> > @@ -102,6 +102,7 @@ eal_long_options[] = {
> >  	{OPT_MATCH_ALLOCATIONS, 0, NULL, OPT_MATCH_ALLOCATIONS_NUM},
> >  	{OPT_TELEMETRY,         0, NULL, OPT_TELEMETRY_NUM        },
> >  	{OPT_NO_TELEMETRY,      0, NULL, OPT_NO_TELEMETRY_NUM     },
> > +	{OPT_FORCE_MAX_SIMD_BITWIDTH, 1, NULL, OPT_FORCE_MAX_SIMD_BITWIDTH_NUM},
> >  	{0,                     0, NULL, 0                        }
> >  };
> > 
> > @@ -1309,6 +1310,34 @@ eal_parse_iova_mode(const char *name)
> >  	return 0;
> >  }
> > 
> > +static int
> > +eal_parse_simd_bitwidth(const char *arg, bool locked)
> > +{
> > +	char *end;
> > +	unsigned long bitwidth;
> > +	int ret;
> > +	struct internal_config *internal_conf =
> > +		eal_get_internal_configuration();
> > +
> > +	if (arg == NULL || arg[0] == '\0')
> > +		return -1;
> > +
> > +	errno = 0;
> > +	bitwidth = strtoul(arg, &end, 0);
> > +
> > +	/* check for errors */
> > +	if (bitwidth > UINT16_MAX || errno != 0 || end == NULL || *end != '\0')
> > +		return -1;
> > +
> > +	if (bitwidth == 0)
> > +		bitwidth = UINT16_MAX;
> > +	ret = rte_set_max_simd_bitwidth(bitwidth);
> > +	if (ret < 0)
> > +		return -1;
> > +	internal_conf->max_simd_bitwidth.locked = locked;
> > +	return 0;
> > +}
> > +
> >  static int
> >  eal_parse_base_virtaddr(const char *arg)
> >  {
> > @@ -1707,6 +1736,13 @@ eal_parse_common_option(int opt, const char *optarg,
> >  	case OPT_NO_TELEMETRY_NUM:
> >  		conf->no_telemetry = 1;
> >  		break;
> > +	case OPT_FORCE_MAX_SIMD_BITWIDTH_NUM:
> > +		if (eal_parse_simd_bitwidth(optarg, 1) < 0) {
> > +			RTE_LOG(ERR, EAL, "invalid parameter for --"
> > +					OPT_FORCE_MAX_SIMD_BITWIDTH "\n");
> > +			return -1;
> > +		}
> > +		break;
> > 
> >  	/* don't know what to do, leave this to caller */
> >  	default:
> > @@ -1903,6 +1939,33 @@ eal_check_common_options(struct internal_config *internal_cfg)
> >  	return 0;
> >  }
> > 
> > +uint16_t
> > +rte_get_max_simd_bitwidth(void)
> > +{
> > +	const struct internal_config *internal_conf =
> > +		eal_get_internal_configuration();
> > +	return internal_conf->max_simd_bitwidth.bitwidth;
> > +}
> > +
> > +int
> > +rte_set_max_simd_bitwidth(uint16_t bitwidth)
> > +{
> > +	struct internal_config *internal_conf =
> > +		eal_get_internal_configuration();
> > +	if (internal_conf->max_simd_bitwidth.locked) {
> > +		RTE_LOG(NOTICE, EAL, "Cannot set max SIMD bitwidth - user runtime override enabled");
> > +		return -EPERM;
> > +	}
> > +
> > +	if (bitwidth != RTE_MAX_SIMD_DISABLE && (bitwidth < RTE_NO_SIMD ||
> > +			!rte_is_power_of_2(bitwidth))) {
> > +		RTE_LOG(ERR, EAL, "Invalid bitwidth value!\n");
> > +		return -EINVAL;
> > +	}
> > +	internal_conf->max_simd_bitwidth.bitwidth = bitwidth;
> > +	return 0;
> > +}
> > +
> >  void
> >  eal_common_usage(void)
> >  {
> > @@ -1981,6 +2044,7 @@ eal_common_usage(void)
> >  	       "  --"OPT_BASE_VIRTADDR"     Base virtual address\n"
> >  	       "  --"OPT_TELEMETRY"   Enable telemetry support (on by default)\n"
> >  	       "  --"OPT_NO_TELEMETRY"   Disable telemetry support\n"
> > +	       "  --"OPT_FORCE_MAX_SIMD_BITWIDTH" Force the max SIMD bitwidth\n"
> >  	       "\nEAL options for DEBUG use only:\n"
> >  	       "  --"OPT_HUGE_UNLINK"       Unlink hugepage files after init\n"
> >  	       "  --"OPT_NO_HUGE"           Use malloc instead of hugetlbfs\n"
> > diff --git a/lib/librte_eal/common/eal_internal_cfg.h b/lib/librte_eal/common/eal_internal_cfg.h
> > index 13f93388a7..367e0cc19e 100644
> > --- a/lib/librte_eal/common/eal_internal_cfg.h
> > +++ b/lib/librte_eal/common/eal_internal_cfg.h
> > @@ -33,6 +33,12 @@ struct hugepage_info {
> >  	int lock_descriptor;    /**< file descriptor for hugepage dir */
> >  };
> > 
> > +struct simd_bitwidth {
> > +	/**< flag indicating if bitwidth is locked from further modification */
> > +	bool locked;
> > +	uint16_t bitwidth; /**< bitwidth value */
> > +};
> > +
> >  /**
> >   * internal configuration
> >   */
> > @@ -85,6 +91,8 @@ struct internal_config {
> >  	volatile unsigned int init_complete;
> >  	/**< indicates whether EAL has completed initialization */
> >  	unsigned int no_telemetry; /**< true to disable Telemetry */
> > +	/** max simd bitwidth path to use */
> > +	struct simd_bitwidth max_simd_bitwidth;
> >  };
> > 
> >  void eal_reset_internal_config(struct internal_config *internal_cfg);
> > diff --git a/lib/librte_eal/common/eal_options.h b/lib/librte_eal/common/eal_options.h
> > index 89769d48b4..ef33979664 100644
> > --- a/lib/librte_eal/common/eal_options.h
> > +++ b/lib/librte_eal/common/eal_options.h
> > @@ -85,6 +85,8 @@ enum {
> >  	OPT_TELEMETRY_NUM,
> >  #define OPT_NO_TELEMETRY      "no-telemetry"
> >  	OPT_NO_TELEMETRY_NUM,
> > +#define OPT_FORCE_MAX_SIMD_BITWIDTH  "force-max-simd-bitwidth"
> > +	OPT_FORCE_MAX_SIMD_BITWIDTH_NUM,
> >  	OPT_LONG_MAX_NUM
> >  };
> > 
> > diff --git a/lib/librte_eal/include/rte_eal.h b/lib/librte_eal/include/rte_eal.h
> > index ddcf6a2e7a..fb739f3474 100644
> > --- a/lib/librte_eal/include/rte_eal.h
> > +++ b/lib/librte_eal/include/rte_eal.h
> > @@ -43,6 +43,14 @@ enum rte_proc_type_t {
> >  	RTE_PROC_INVALID
> >  };
> > 
> > +enum rte_max_simd_t {
> > +	RTE_NO_SIMD = 64,
> 
> While I do understand the idea of having that value from consistency point of view,
> I wonder do we really need to allow user to specify values smaller then 128.
> At least on x86 we always have 128 bit SIMD enabled, even for -Dmachine=default.
> So seems no much point to forbid libraries using SSE code-path when compiler
> is free to insert SSE instructions on its own will.  
> 

The reason to support this is for testing purposes, as it allows an easy
way for a tester to check out any scalar code paths - which are often
common across architectures.

> > +	RTE_MAX_128_SIMD = 128,
> > +	RTE_MAX_256_SIMD = 256,
> > +	RTE_MAX_512_SIMD = 512,
> > +	RTE_MAX_SIMD_DISABLE = UINT16_MAX,
> 
> As a nit, I think it is safe enough to have this last value 
> (RTE_MAX_SIMD_DISABLE or RTE_MAX_SIMD_MAX) equal to (INT16_MAX + 1).
> That would be big enough to probably never hit actual HW limit,
> while it still remains power of two, as other values. 
> 

I actually think it's probably clearer as-is, because the fact of the rest
being powers of 2 is irrelevant since we just check greater than or less
than. If we did change it, then we need to put in a comment explaining why
the plus-one, while as it is now it's clearly a placeholder $BIGNUM.

/Bruce