linux-kernel - Re: [PATCH v5 1/3] selftests/lam: Move cpu_has

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Message-ID: <e1aefeca-8f85-48c9-8972-1c23b34aea7b@intel.com>
Date: Fri, 24 Jan 2025 08:14:41 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>,
 shuah@...nel.org, hpa@...or.com, x86@...nel.org,
 dave.hansen@...ux.intel.com, bp@...en8.de, mingo@...hat.com,
 tglx@...utronix.de
Cc: linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
 kirill@...temov.name, "Kirill A. Shutemov"
 <kirill.shutemov@...ux.intel.com>, Shuah Khan <skhan@...uxfoundation.org>
Subject: Re: [PATCH v5 1/3] selftests/lam: Move cpu_has_la57() to use cpuinfo
 flag

On 11/27/24 09:35, Maciej Wieczor-Retman wrote:
> -/* Check 5-level page table feature in CPUID.(EAX=07H, ECX=00H):ECX.[bit 16] */
>  static inline int cpu_has_la57(void)
>  {
> -	unsigned int cpuinfo[4];
> -
> -	__cpuid_count(0x7, 0, cpuinfo[0], cpuinfo[1], cpuinfo[2], cpuinfo[3]);
> -
> -	return (cpuinfo[2] & (1 << 16));
> +	return !system("grep -wq la57 /proc/cpuinfo");
>  }

I would rather we find another way.

First, we've documented the behavior a bit in here:

	https://docs.kernel.org/arch/x86/cpuinfo.html

The important part is:

	"The absence of a flag in /proc/cpuinfo by itself means almost
	nothing to an end user."

Even worse, let's say there's a CPU bug and we say define a bug bit:

	bugs		: spectre_v1 spectre_v2 ... la57_is_broken

How is that grep going to work out? ;)

Could you poke around and see if there is any existing ABI that we can
use to query LA57 support? Maybe one of the things KVM exports, or some
TASK_SIZE_MAX comparisons?