linux-kernel - Re: [patch V4 02/41] cpu/SMT: Make SMT control more robust against enumeration failures

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Message-ID: <3b9518f5-8b92-1c24-286c-47bf9e8b8a2e@intel.com>
Date:   Tue, 15 Aug 2023 14:15:53 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     x86@...nel.org, Tom Lendacky <thomas.lendacky@....com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Huang Rui <ray.huang@....com>, Juergen Gross <jgross@...e.com>,
        Dimitri Sivanich <dimitri.sivanich@....com>,
        Michael Kelley <mikelley@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>, Pu Wen <puwen@...on.cn>,
        Qiuxu Zhuo <qiuxu.zhuo@...el.com>,
        Sohil Mehta <sohil.mehta@...el.com>
Subject: Re: [patch V4 02/41] cpu/SMT: Make SMT control more robust against
 enumeration failures

On 8/14/23 01:53, Thomas Gleixner wrote:
> -static inline bool cpu_smt_allowed(unsigned int cpu)
> +static inline bool cpu_bootable(unsigned int cpu)
>  {
>  	if (cpu_smt_control == CPU_SMT_ENABLED)
>  		return true;
>  
> +	if (cpu_smt_control == CPU_SMT_NOT_SUPPORTED)
> +		return true;

I found this new pair of if()'s rather counterintuitive to read.

The first one reads like:

	"If SMT is not supported, the CPU is always bootable"

but "supported" could easily mean CONFIG_SMP==n (which is actually
covered in the next case).  Would this be better named:

	CPU_SMT_NOT_ENUMERATED
or
	CPU_SMT_NOT_DETECTED

?

	/* Every CPU is bootable on non-SMT systems: */
	if (cpu_smt_control == CPU_SMT_NOT_DETECTED)
		return true;

For the next one:

> +	if (cpu_smt_control == CPU_SMT_NOT_IMPLEMENTED)
> +		return true;

This reads a bit like "SMT is not implemented" rather than "SMT controls
are not implemented".  Maybe a comment would help:

	/* All CPUs are bootable if controls are not implemented: */