Re: [RFC PATCH 06/12] perf: Support extension of sample_regs

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Jun 13, 2025 at 06:49:37AM -0700, kan.liang@linux.intel.com wrote:
> From: Kan Liang <kan.liang@linux.intel.com>
> 
> More regs may be required in a sample, e.g., the vector registers. The
> current sample_regs_XXX has run out of space.
> 
> Add sample_ext_regs_intr/user[2] in the struct perf_event_attr. It's used
> as a bitmap for the extension regs. There will be more than 64 registers
> added.
> Add a new flag PERF_PMU_CAP_EXTENDED_REGS2 to indicate the PMU which
> supports sample_ext_regs_intr/user.
> 
> Extend the perf_reg_validate() to support the validation of the
> extension regs.
> 
> Extend the perf_reg_value() to retrieve the extension regs. The regs may
> be larger than u64. Add two parameters to store the pointer and size.
> Add a dedicated perf_output_sample_ext_regs() to dump the extension
> regs.
> 
> This is just a generic support for the extension regs. Any attempts to
> manipulate the extension regs will error out, until the driver-specific
> supports are implemented, which will be done in the following patch.
> 

> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> index 78a362b80027..e22ba72efcdb 100644
> --- a/include/uapi/linux/perf_event.h
> +++ b/include/uapi/linux/perf_event.h
> @@ -382,6 +382,10 @@ enum perf_event_read_format {
>  #define PERF_ATTR_SIZE_VER6			120	/* Add: aux_sample_size */
>  #define PERF_ATTR_SIZE_VER7			128	/* Add: sig_data */
>  #define PERF_ATTR_SIZE_VER8			136	/* Add: config3 */
> +#define PERF_ATTR_SIZE_VER9			168	/* Add: sample_ext_regs_intr */
> +							/* Add: sample_ext_regs_user */
> +
> +#define PERF_ATTR_EXT_REGS_SIZE			2
>  
>  /*
>   * 'struct perf_event_attr' contains various attributes that define
> @@ -543,6 +547,10 @@ struct perf_event_attr {
>  	__u64	sig_data;
>  
>  	__u64	config3; /* extension of config2 */
> +
> +	/* extension of sample_regs_XXX */
> +	__u64	sample_ext_regs_intr[PERF_ATTR_EXT_REGS_SIZE];
> +	__u64	sample_ext_regs_user[PERF_ATTR_EXT_REGS_SIZE];
>  };

Did anybody read this email?

  https://lkml.kernel.org/r/20250416155327.GD17910@noisy.programming.kicks-ass.net

The current regs interface really was designed for regular registers,
and trying to squish SIMD registers into it is a trainwreck.

Not to mention that AAAARGHH64 and Risc-V have vector widths up to 2048
bit.