linux-kernel - Re: x86/apic: Drop useless CXL/CPX row from the TSC deadline errata table

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Message-ID: <11530e8c-2e91-4071-8fd2-7657e2f836af@intel.com>
Date: Fri, 18 Jul 2025 12:25:00 -0700
From: Sohil Mehta <sohil.mehta@...el.com>
To: Andrew Cooper <andrew.cooper3@...rix.com>
CC: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
	<x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>, Peter Zijlstra
	<peterz@...radead.org>, Yuntao Wang <yuntao.wang@...ux.dev>, Zhang Rui
	<rui.zhang@...el.com>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: x86/apic: Drop useless CXL/CPX row from the TSC deadline errata
 table

On 7/18/2025 3:43 AM, Andrew Cooper wrote:
> On 17/07/2025 9:25 pm, Sohil Mehta wrote:
>> On 7/16/2025 9:08 AM, Andrew Cooper wrote:
>>> A microcode revision of 0 is guaranteed to exit apic_validate_deadline_timer()
>>> early, but a better way is with no row at all.
>>>
>> Yeah, not having an entry is much better. "exit" can be a bit ambiguous.
>> Should we be slightly more precise?
> 
> It's "exit $foo early", although this is slightly hidden by the linebreak.
> 

Sorry, I was being too finicky.

I meant that a microcode revision of 0 would cause
apic_validate_deadline_timer() to exit with the return value of true.

	if (boot_cpu_data.microcode >= rev)
		return true;

Not having a row as well would return true because of:

	m = x86_match_cpu(deadline_match);
	if (!m)
		return true;

I wanted to highlight that apic_validate_deadline_timer() would return
*true* in both cases, rather than just "exit" early.

What you have right now is okay as well :)

> Alternatively, it could be phrased as "exit early from $foo" if you
> think that's clearer?
> 
>>
>> A microcode revision of 0 is guaranteed to return true for
>> apic_validate_deadline_timer(), but a better way is with no row at all.
>>

Sohil