linux-kernel - Re: [PATCH v2 06/10] x86/mce: Convert multiple if () statements into a switch() statement

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Message-ID: <7a902c13-bfdf-4319-9e31-81c199ecf65c@intel.com>
Date: Mon, 21 Oct 2024 11:26:02 -0700
From: Sohil Mehta <sohil.mehta@...el.com>
To: "Luck, Tony" <tony.luck@...el.com>, "Zhuo, Qiuxu" <qiuxu.zhuo@...el.com>
CC: "bp@...en8.de" <bp@...en8.de>, "tglx@...utronix.de" <tglx@...utronix.de>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
	"mingo@...hat.com" <mingo@...hat.com>, "hpa@...or.com" <hpa@...or.com>,
	"x86@...nel.org" <x86@...nel.org>, "linux-edac@...r.kernel.org"
	<linux-edac@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 06/10] x86/mce: Convert multiple if () statements into
 a switch() statement

On 10/21/2024 10:51 AM, Luck, Tony wrote:
>> But looking at the prior or the later checks, I see the '<' operator
>> used directly on platform names. So, the new check seems inline with
>> that i.e. in this case, any model or family after the said platform
>> supports MCE broadcasting.
> 
> Intel model number allocation policies aren't necessarily sequential.

Model numbers are assumed to be sequential at least within family 6.

if ((c->x86 > 6 || (c->x86 == 6 && c->x86_model >= 0xe)) &&
	cfg->monarch_timeout < 0)

There is another check in early_init_intel() which does this:

	if (c->x86 > 6 || (c->x86 == 6 && c->x86_model >= 0xd)) {
		rdmsrl(MSR_IA32_MISC_ENABLE, misc_enable);


> So range checks need to be used with caution. They should be safe
> enough when done to simplify code that checks very old models.
> 

...

> Range checks across families may be even more problematic. 

I agree. Maybe we should avoid range checks across families altogether
forward (>) or backward (<).

For example, does the following change from Qiuxu, unintentionally
become applicable to Quark CPUs with family -> 5?


>         /*
>          * There are also broken BIOSes on some Pentium M and
>          * earlier systems:
>          */
> -       if (c->x86 == 6 && c->x86_model <= 13 && cfg->bootlog < 0)
> +       if (c->x86_vfm < INTEL_CORE_YONAH && cfg->bootlog < 0)
>                 cfg->bootlog = 0;