linux-kernel - Re: [PATCH v2] hwmon: (dell-smm) Improve assembly code

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Date:   Sun, 20 Feb 2022 20:05:17 +0100
From:   Armin Wolf <W_Armin@....de>
To:     David Laight <David.Laight@...LAB.COM>,
        "pali@...nel.org" <pali@...nel.org>
Cc:     "jdelvare@...e.com" <jdelvare@...e.com>,
        "linux@...ck-us.net" <linux@...ck-us.net>,
        "linux-hwmon@...r.kernel.org" <linux-hwmon@...r.kernel.org>,
        "linux-assembly@...r.kernel.org" <linux-assembly@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] hwmon: (dell-smm) Improve assembly code

Am 20.02.22 um 19:48 schrieb David Laight:

> From: Armin Wolf
>> Sent: 19 February 2022 21:10
>>
>> The new assembly code works on both 32 bit and 64 bit
>> cpus and allows for more compiler optimisations by not
>> requiring smm_regs to be packed
> I'm intrigued about the __packed..
>
> Prior to 5.17-rc1 __packed was only applied to the fields after 'eax'.
> This actually has no effect (on any architecture) because there is
> no padding to remove - so all the later fields are still assumed to
> be 32bit aligned.
>
> 5.17-rc1 (565210c781201) moved the __packed to the end of the
> structure.
> AFAICT this structure is only ever used in one file and for on-stack
> items. It will always actually be aligned and is only read by the
> code in the file - so why was it ever marked __packed at all!
> On x86 it would make no difference anyway.
>
> I can only guess it was to ensure that the asm code didn't go
> 'wrong' because of the compiler adding 'random' padding.
> That isn't what __packed is for at all.
> The linux kernel requires that the compiler doesn't add 'random'
> padding - even if the C standard might allow it.
>
> 	David
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)
>
Your right, after looking at the assembly output, i can confirm that
the removal of __packed changed nothing.
Maybe i should update this part of the commit message as well.

Armin Wolf