linux-kernel - Re: [PATCH 2/4] perf/x86/ibs: Use CPUID region helper

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Message-ID: <20240416151242.GGZh6VaiO2gC4ej2BT@fat_crate.local>
Date: Tue, 16 Apr 2024 17:12:42 +0200
From: Borislav Petkov <bp@...en8.de>
To: Dave Hansen <dave.hansen@...ux.intel.com>,
	Robert Richter <rric@...nel.org>
Cc: linux-kernel@...r.kernel.org, jgross@...e.com, tglx@...utronix.de,
	x86@...nel.org
Subject: Re: [PATCH 2/4] perf/x86/ibs: Use CPUID region helper

On Wed, Apr 03, 2024 at 08:35:11AM -0700, Dave Hansen wrote:
> 
> From: Dave Hansen <dave.hansen@...ux.intel.com>
> 
> IBS details are enumerated in an extended CPUID leaf.  But
> the support has an open-coded CPUID region check.  Use the
> new helper to trim down the code.
> 
> Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
> --
> 
> Note: this cleanup could take another form:
> 
>         if (boot_cpu_data->extended_cpuid_level >= IBS_CPUID_FEATURES)
>                 caps = cpuid_eax(IBS_CPUID_FEATURES);
> 
> that would be one fewer CPUID invocations, but one more
> line of code.
> ---
> 
>  b/arch/x86/events/amd/ibs.c |    9 ++-------
>  1 file changed, 2 insertions(+), 7 deletions(-)
> 
> diff -puN arch/x86/events/amd/ibs.c~ibs-region-helpers arch/x86/events/amd/ibs.c
> --- a/arch/x86/events/amd/ibs.c~ibs-region-helpers	2024-04-02 15:22:59.262912595 -0700
> +++ b/arch/x86/events/amd/ibs.c	2024-04-02 15:22:59.262912595 -0700
> @@ -1278,18 +1278,13 @@ static __init int perf_event_ibs_init(vo
>  
>  static __init u32 __get_ibs_caps(void)
>  {
> -	u32 caps;
> -	unsigned int max_level;
> +	u32 caps = 0;
>  
>  	if (!boot_cpu_has(X86_FEATURE_IBS))
>  		return 0;
>  
> -	/* check IBS cpuid feature flags */
> -	max_level = cpuid_eax(0x80000000);
> -	if (max_level < IBS_CPUID_FEATURES)
> -		return IBS_CAPS_DEFAULT;
> +	get_cpuid_region_leaf(IBS_CPUID_FEATURES, CPUID_EAX, &caps);

I wanna say all this checking of max level is worthless because if you
have X86_FEATURE_IBS, then it is a given that you also have that
0x8000001b CPUID leaf.

Right, Bob?

Unless there was some weird thing back then with the CPUID leafs...

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette