linux-kernel - Re: [PATCH v6 08/11] misc: amd-sbi: Add support for CPUID protocol

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Message-Id: <3ab1116b-7021-4d83-847d-fdec53af9bee@app.fastmail.com>
Date: Mon, 24 Mar 2025 16:29:20 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: "Akshay Gupta" <akshay.gupta@....com>, linux-hwmon@...r.kernel.org,
 linux-kernel@...r.kernel.org
Cc: "Guenter Roeck" <linux@...ck-us.net>,
 "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>, shyam-sundar.s-k@....com,
 gautham.shenoy@....com, "Mario Limonciello" <mario.limonciello@....com>,
 naveenkrishna.chatradhi@....com, anand.umarji@....com
Subject: Re: [PATCH v6 08/11] misc: amd-sbi: Add support for CPUID protocol

On Mon, Mar 24, 2025, at 15:58, Akshay Gupta wrote:

>  struct apml_message {
>  	/*
>  	 * [0]...[3] mailbox 32bit input
> +	 *	     cpuid,
> +	 * [4][5] cpuid: thread
> +	 * [6] cpuid: ext function & read eax/ebx or ecx/edx
> +	 *	[7:0] -> bits [7:4] -> ext function &
> +	 *	bit [0] read eax/ebx or ecx/edx
>  	 * [7] read/write functionality
>  	 */
>  	union {
> +		__u64 cpu_msr_in;
>  		__u32 mb_in[2];
>  		__u8 reg_in[8];
>  	} data_in;

This changes the alignment of the structure and makes
it incompatible between 32-bit and 64-bit userspace
when there is an odd number of 32-bit words in it.

      Arnd