linux-kernel - Re: [Patch v5 07/19] perf: Add sampling support for SIMD registers

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Message-ID: <20251205110706.GT2528459@noisy.programming.kicks-ass.net>
Date: Fri, 5 Dec 2025 12:07:06 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Dapeng Mi <dapeng1.mi@...ux.intel.com>
Cc: Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Jiri Olsa <jolsa@...nel.org>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Eranian Stephane <eranian@...gle.com>,
	Mark Rutland <mark.rutland@....com>, broonie@...nel.org,
	Ravi Bangoria <ravi.bangoria@....com>, linux-kernel@...r.kernel.org,
	linux-perf-users@...r.kernel.org, Zide Chen <zide.chen@...el.com>,
	Falcon Thomas <thomas.falcon@...el.com>,
	Dapeng Mi <dapeng1.mi@...el.com>, Xudong Hao <xudong.hao@...el.com>,
	Kan Liang <kan.liang@...ux.intel.com>
Subject: Re: [Patch v5 07/19] perf: Add sampling support for SIMD registers

On Wed, Dec 03, 2025 at 02:54:48PM +0800, Dapeng Mi wrote:

> @@ -545,6 +547,25 @@ struct perf_event_attr {
>  	__u64	sig_data;
>  
>  	__u64	config3; /* extension of config2 */
> +
> +
> +	/*
> +	 * Defines set of SIMD registers to dump on samples.
> +	 * The sample_simd_regs_enabled !=0 implies the
> +	 * set of SIMD registers is used to config all SIMD registers.
> +	 * If !sample_simd_regs_enabled, sample_regs_XXX may be used to
> +	 * config some SIMD registers on X86.
> +	 */
> +	union {
> +		__u16 sample_simd_regs_enabled;
> +		__u16 sample_simd_pred_reg_qwords;
> +	};
> +	__u32 sample_simd_pred_reg_intr;
> +	__u32 sample_simd_pred_reg_user;
> +	__u16 sample_simd_vec_reg_qwords;
> +	__u64 sample_simd_vec_reg_intr;
> +	__u64 sample_simd_vec_reg_user;
> +	__u32 __reserved_4;
>  };

This is poorly aligned and causes holes.

This:

diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
index d292f96bc06f..2deb8dd0ca37 100644
--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h
@@ -545,6 +545,14 @@ struct perf_event_attr {
 	__u64	sig_data;
 
 	__u64	config3; /* extension of config2 */
+
+	__u16	sample_simd_pred_reg_qwords;
+	__u32	sample_simd_pred_reg_intr;
+	__u32	sample_simd_pred_reg_user;
+	__u16	sample_simd_vec_reg_qwords;
+	__u64	sample_simd_vec_reg_intr;
+	__u64	sample_simd_vec_reg_user;
+	__u32	__reserved_4;
 };
 
 /*

results in:

        __u64                      config3;              /*   128     8 */
        __u16                      sample_simd_pred_reg_qwords; /*   136     2 */

        /* XXX 2 bytes hole, try to pack */

        __u32                      sample_simd_pred_reg_intr; /*   140     4 */
        __u32                      sample_simd_pred_reg_user; /*   144     4 */
        __u16                      sample_simd_vec_reg_qwords; /*   148     2 */

        /* XXX 2 bytes hole, try to pack */

        __u64                      sample_simd_vec_reg_intr; /*   152     8 */
        __u64                      sample_simd_vec_reg_user; /*   160     8 */
        __u32                      __reserved_4;         /*   168     4 */



A better layout might be:

diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
index d292f96bc06f..f72707e9df68 100644
--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h
@@ -545,6 +545,15 @@ struct perf_event_attr {
 	__u64	sig_data;
 
 	__u64	config3; /* extension of config2 */
+
+	__u16	sample_simd_pred_reg_qwords;
+	__u16	sample_simd_vec_reg_qwords;
+	__u32	__reserved_4;
+
+	__u32	sample_simd_pred_reg_intr;
+	__u32	sample_simd_pred_reg_user;
+	__u64	sample_simd_vec_reg_intr;
+	__u64	sample_simd_vec_reg_user;
 };
 
 /*

such that:

        __u64                      config3;              /*   128     8 */
        __u16                      sample_simd_pred_reg_qwords; /*   136     2 */
        __u16                      sample_simd_vec_reg_qwords; /*   138     2 */
        __u32                      __reserved_4;         /*   140     4 */
        __u32                      sample_simd_pred_reg_intr; /*   144     4 */
        __u32                      sample_simd_pred_reg_user; /*   148     4 */
        __u64                      sample_simd_vec_reg_intr; /*   152     8 */
        __u64                      sample_simd_vec_reg_user; /*   160     8 */