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

[Peter Zijlstra]

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.