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

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Message-ID: <20221007203425.GA5647@ranerica-svr.sc.intel.com>
Date:   Fri, 7 Oct 2022 13:34:25 -0700
From:   Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 Tue, Sep 27, 2022 at 01:46:59PM +0200, Peter Zijlstra wrote:
> 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.

I checked the C99 and C11 specs and, IIUC, it does prescribe how to handle
adjacent bit-fields:

	"An implementation may allocate any addressable storage unit large
	 enough to hold a bitfield. If enough space remains, a bit-field
	 that immediately follows another bit-field in a structure shall be
	 packed into adjacent bits of the same unit."

Hence, __reserved and valid should be packed. classid, however, it is not
guaranteed to be adjacent to __reserved.

I will implement the struct are you have described.

Thanks and BR,
Ricardo