linux-kernel - Re: [PATCH v4 1/3] selftests/lam: Move cpu_has

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Message-ID: <6kfafs7wio7ruth3p54pezqwcultxqqpnjvehjzaz7hlba4rp3@6kb5zdqfglsl>
Date: Wed, 27 Nov 2024 14:11:38 +0200
From: "Kirill A. Shutemov" <kirill@...temov.name>
To: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
Cc: shuah@...nel.org, hpa@...or.com, x86@...nel.org, 
	dave.hansen@...ux.intel.com, bp@...en8.de, mingo@...hat.com, tglx@...utronix.de, 
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v4 1/3] selftests/lam: Move cpu_has_la57() to use cpuinfo
 flag

On Tue, Nov 26, 2024 at 02:34:48PM +0100, Maciej Wieczor-Retman wrote:
> In current form cpu_has_la57() reports platform's support for LA57
> through reading the output of cpuid. A much more useful information is
> whether 5-level paging is actually enabled on the running system.
> 
> Presence of the la57 flag in /proc/cpuinfo signifies that 5-level paging
> was compiled into the kernel, is supported by the platform and wasn't
> disabled by kernel command line argument.
> 
> Use system() with cat and grep to figure out if la57 is enabled on the
> running system.
> 
> Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
> ---
> Changelog v4:
> - Add this patch to the series.
> 
>  tools/testing/selftests/x86/lam.c | 7 ++-----
>  1 file changed, 2 insertions(+), 5 deletions(-)
> 
> diff --git a/tools/testing/selftests/x86/lam.c b/tools/testing/selftests/x86/lam.c
> index 0ea4f6813930..0ac805125ab2 100644
> --- a/tools/testing/selftests/x86/lam.c
> +++ b/tools/testing/selftests/x86/lam.c
> @@ -124,14 +124,11 @@ static inline int cpu_has_lam(void)
>  	return (cpuinfo[0] & (1 << 26));
>  }
>  
> -/* 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]);
> +	int ret = !!system("cat /proc/cpuinfo | grep -wq la57\n");

Heh. grep can read files on its own :P

	return !system("grep -wq la57 /proc/cpuinfo");

>  
> -	return (cpuinfo[2] & (1 << 16));
> +	return !ret;
>  }
>  
>  /*
> -- 
> 2.47.1
> 

-- 
  Kiryl Shutsemau / Kirill A. Shutemov