linux-kernel - Re: [RFC PATCH 00/12] Support vector and more extended registers in perf

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Message-ID: <20250619133823.GG1613376@noisy.programming.kicks-ass.net>
Date: Thu, 19 Jun 2025 15:38:23 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: "Liang, Kan" <kan.liang@...ux.intel.com>
Cc: "Mi, Dapeng" <dapeng1.mi@...ux.intel.com>,
	Mark Rutland <mark.rutland@....com>, mingo@...hat.com,
	acme@...nel.org, namhyung@...nel.org, tglx@...utronix.de,
	dave.hansen@...ux.intel.com, irogers@...gle.com,
	adrian.hunter@...el.com, jolsa@...nel.org,
	alexander.shishkin@...ux.intel.com, linux-kernel@...r.kernel.org,
	ak@...ux.intel.com, zide.chen@...el.com
Subject: Re: [RFC PATCH 00/12] Support vector and more extended registers in
 perf

On Thu, Jun 19, 2025 at 07:11:23AM -0400, Liang, Kan wrote:

> @@ -543,6 +544,24 @@ struct perf_event_attr {
>  	__u64	sig_data;
> 
>  	__u64	config3; /* extension of config2 */
> +
> +
> +	/*
> +	 * Defines set of SIMD registers to dump on samples.
> +	 * The sample_simd_req_enabled !=0 implies the
> +	 * set of SIMD registers is used to config all SIMD registers.
> +	 * If !sample_simd_req_enabled, sample_regs_XXX may be used to
> +	 * config some SIMD registers on X86.
> +	 */
> +	union {
> +		__u16 sample_simd_reg_enabled;
> +		__u16 sample_simd_pred_reg_qwords;
> +	};
> +	__u16 sample_simd_pred_reg_intr;
> +	__u16 sample_simd_pred_reg_user;

This limits things to max 16 predicate registers. ARM will fully fill
that with present hardware.

> +	__u16 sample_simd_reg_qwords;
> +	__u64 sample_simd_reg_intr;
> +	__u64 sample_simd_reg_user;

I would perhaps make this vec_reg.

>  };
> 
>  /*
> @@ -1016,7 +1035,15 @@ enum perf_event_type {
>  	 *      } && PERF_SAMPLE_BRANCH_STACK
>  	 *
>  	 *	{ u64			abi; # enum perf_sample_regs_abi
> -	 *	  u64			regs[weight(mask)]; } && PERF_SAMPLE_REGS_USER
> +	 *	  u64			regs[weight(mask)];
> +	 *	  struct {
> +	 *	  	u16 nr_vectors;
> +	 *	  	u16 vector_qwords;
> +	 *	  	u16 nr_pred;
> +	 *	  	u16 pred_qwords;
> +	 *	  	u64 data[nr_vectors * vector_qwords + nr_pred * pred_qwords];
> +	 *	  } && sample_simd_reg_enabled

Instead of using sample_simd_reg_enabled here I would perhaps extend
perf_sample_regs_abi. The current abi word is woefully underused.

Also, realistically, what you want to look at here is:

  sample_simd_{pred,vec}_reg_user;

If those are empty, there will be no registers.

> +	 *	} && PERF_SAMPLE_REGS_USER
>  	 *
>  	 *	{ u64			size;
>  	 *	  char			data[size];