linux-kernel - Re: [PATCH v3 4/4] x86/CPU/AMD: Print the reason for the last reset

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Message-ID: <42b7547d-c1f7-4509-a381-7bf0a485a5f5@kernel.org>
Date: Fri, 11 Apr 2025 07:12:24 -0500
From: Mario Limonciello <superm1@...nel.org>
To: Borislav Petkov <bp@...en8.de>
Cc: Jean Delvare <jdelvare@...e.com>, Andi Shyti <andi.shyti@...nel.org>,
 Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
 Jonathan Corbet <corbet@....net>,
 Mario Limonciello <mario.limonciello@....com>,
 Yazen Ghannam <yazen.ghannam@....com>, Thomas Gleixner <tglx@...utronix.de>,
 Ingo Molnar <mingo@...hat.com>, Dave Hansen <dave.hansen@...ux.intel.com>,
 "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
 "H . Peter Anvin" <hpa@...or.com>,
 Shyam Sundar S K <Shyam-sundar.S-k@....com>,
 Hans de Goede <hdegoede@...hat.com>,
 "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
 open list <linux-kernel@...r.kernel.org>,
 "open list:I2C/SMBUS CONTROLLER DRIVERS FOR PC" <linux-i2c@...r.kernel.org>,
 "open list:AMD PMC DRIVER" <platform-driver-x86@...r.kernel.org>
Subject: Re: [PATCH v3 4/4] x86/CPU/AMD: Print the reason for the last reset

On 4/11/25 07:06, Borislav Petkov wrote:
> On Thu, Apr 10, 2025 at 03:02:02PM -0500, Mario Limonciello wrote:
>> +static __init int print_s5_reset_status_mmio(void)
>> +{
>> +	void __iomem *addr;
>> +	unsigned long value;
>> +	int bit = -1;
>> +
>> +	if (!cpu_feature_enabled(X86_FEATURE_ZEN))
>> +		return 0;
>> +
>> +	addr = ioremap(FCH_PM_BASE + FCH_PM_S5_RESET_STATUS, sizeof(value));
>> +	if (!addr)
>> +		return 0;
> 
> newline.
> 
>> +	value = ioread32(addr);
>> +	iounmap(addr);
>> +
>> +	do {
>> +		bit = find_next_bit(&value, BITS_PER_LONG, bit + 1);
>> +	} while (!s5_reset_reason_txt[bit]);
> 
> What's the idea here? The highest bit is the most fitting one?
> 
> So why don't you do fls() or so?

The idea was to walk all the bits and pick the first one that has a 
string associated with it.  I was finding that sometimes the reserved 
bits are set which would get you a NULL pointer deref.

> 
>> +	pr_info("x86/amd: Previous system reset reason [0x%08lx]: %s\n",
>> +		value, s5_reset_reason_txt[bit]);
> 
> What's guaranteeing that s5_reset_reason_txt[bit] is still set here?
> 
> I'd suggest you check it again and never trust the hw because we'll be fixing
> a null ptr here at some point otherwise...
> 

Right; I was worried about that too but find_next_bit() will return the 
size argument when it doesn't find anything.

So that should be s5_reset_reason_txt[32] which has the "Unknown" string.