linux-kernel - Re: [RFC PATCH 14/23] thermal: intel: hfi: Update the class of the current task

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Message-ID: <YzLis/a3aIJnJ2ZE@hirez.programming.kicks-ass.net>
Date:   Tue, 27 Sep 2022 13:46:59 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
Cc:     Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Ricardo Neri <ricardo.neri@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Len Brown <len.brown@...el.com>, Mel Gorman <mgorman@...e.de>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Valentin Schneider <vschneid@...hat.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, "Tim C . Chen" <tim.c.chen@...el.com>
Subject: Re: [RFC PATCH 14/23] thermal: intel: hfi: Update the class of the
 current task

On Fri, Sep 09, 2022 at 04:11:56PM -0700, Ricardo Neri wrote:
> +union hfi_thread_feedback_char_msr {
> +	struct {
> +		u8	classid;
> +		u64	__reserved:55;
> +		u8	valid:1;
> +	} split;
> +	u64 full;
> +};

Urgh, did you perhaps mean:

	struct {
		u64	classid    :8;
		u64	__reserved :55;
		u64	valid      :1
	};

?

Because yes, GCC does fold that into a single u64, but that's
implementation defined behaviour; the C spec doesn't require one to pack
adjacent bitfields of different types together.

I layout of:

	u8	class;			// offset 0
	u64	__reserver : 55;	// offset 8
	u8	valid : 1;		// offset 16

with a total size of 24 bytes is, AFAIU, a valid result of what you
wrote.