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

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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

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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




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