linux-kernel - Re: [RFC PATCH 08/12] perf/x86: Add APX in extended regs

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Message-ID: <9624b098-f3ab-4db7-8811-cab2c5cce351@intel.com>
Date: Fri, 13 Jun 2025 09:02:25 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: kan.liang@...ux.intel.com, peterz@...radead.org, 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
Cc: dapeng1.mi@...ux.intel.com, ak@...ux.intel.com, zide.chen@...el.com
Subject: Re: [RFC PATCH 08/12] perf/x86: Add APX in extended regs

On 6/13/25 06:49, kan.liang@...ux.intel.com wrote:
> +#define __x86_pmu_get_regs(_mask, _regs, _size)		\
> +do {							\
> +	if (mask & _mask && xcomp_bv & _mask) {		\
> +		_regs = xsave;				\
> +		xsave += _size;				\
> +	}						\
> +} while (0)

Ewww.

First of all, this doesn't work generally because of the previously
mentioned alignment. Second, it's using xcomp_bv which doesn't tell you
if XSAVES wrote the data.

Last, this attempts to reimplement get_xsave_addr().

I'd do something like this:

	for (xfeature_nr in mask) {
		void *src = get_xsave_addr(xsave, xfeature_nr);
		void *dst = ... a function to map XFEATURE_MASK_APX
				to perf_regs->apx_regs,
		int size  = xstate_sizes(xfeature_nr);

		if (!src)
			continue;

		memcpy(dst, src, size);
	}

That should handle *all* of the nastiness. The alignment, the init
optimization. *Please* use get_xsave_addr() or one of the other helpers.