linux-kernel - Re: [PATCH v4] hwmon: (dell-smm) Improve assembly code

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Date:   Wed, 23 Feb 2022 16:45:27 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     David Laight <David.Laight@...LAB.COM>,
        'Armin Wolf' <W_Armin@....de>,
        'Pali Rohár' <pali@...nel.org>
Cc:     "jdelvare@...e.com" <jdelvare@...e.com>,
        "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 v4] hwmon: (dell-smm) Improve assembly code

On 2/23/22 14:35, David Laight wrote:
> From: Armin Wolf
>> Sent: 23 February 2022 21:39
> ...
>>> As far as this patch goes I think I'd add a 'stc' (set carry)
>>> instruction before the first 'outb'.
>>> Then if the 'something magic happens here' doesn't happen (because
>>> you aren't on the right kind of motherboard) the action fails.
>>>
>>> 	David
>>
>> We already check for such scenarios by checking if eax changed.
>> I would not set the carry flag before doing a SMM call since im
>> not sure if the firmware code always clears the carry flag when
>> the call was successful.
>> If for example the firmware code only sets the carry flag on
>> error and otherwise ignores the carry flag, we might create some
>> false negatives when a successful SMM call leaves the carry flag set.
> 
> If you are worried about that you should be clearing carry on entry.
> 

I agree, but now we have an argument for clearing carry (because the
SMM code might not do it) and for setting carry (because the SMM code
might not execute). I don't know which way to go, if any, but that
would be a functional change and should be submitted as separate patch.

Guenter