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

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Date:   Sun, 20 Feb 2022 18:48:58 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Armin Wolf' <W_Armin@....de>, "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

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

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