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

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Date: Mon, 22 Apr 2024 19:20:55 +0200
From: Borislav Petkov <bp@...en8.de>
To: Robert Richter <rrichter@....com>
Cc: Dave Hansen <dave.hansen@...el.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	linux-kernel@...r.kernel.org, jgross@...e.com, tglx@...utronix.de,
	x86@...nel.org, Kim Phillips <kim.phillips@....com>,
	Robert Richter <rric@...nel.org>
Subject: Re: [PATCH 2/4] perf/x86/ibs: Use CPUID region helper

On Thu, Apr 18, 2024 at 02:05:49PM +0200, Robert Richter wrote:
> There is the X86_FEATURE_IBS bit (Fn8000_0001_ECX, bit 10) which is
> available from the beginning of IBS (all Fam10h production releases,
> revB onwards).
> 
> And right, IBS_CPUID_FEATURES (CPUID Fn8000_001B) was introduced with
> revC. The capabilities of revB are set in IBS_CAPS_DEFAULT.
> 
> It doesn't look broken to me, simply the ibs caps field was introduced
> later which can be determined checking the return code of
> get_cpuid_region_leaf().

Right.

> My preference would be:
> 
> 	[...]
> 	if (!get_cpuid_region_leaf(IBS_CPUID_FEATURES, CPUID_EAX, &caps))

Right, checking get_cpuid_region_leaf() retval should happen.

> 		return IBS_CAPS_DEFAULT;
> 
> 	if (caps & IBS_CAPS_AVAIL)
> 		return caps;
> 
> 	return 0;
> 	[...]
> 
> Not too complex?

I'm wondering should we even support those old things? All revBs should
be probably dead by now. But ok, it's not like it is a lot of code or
so...

> This slightly modifies the functionality so that 0 is return if
> !IBS_CAPS_AVAIL (instead of IBS_CAPS_DEFAULT).

If !IBS_CAPS_AVAIL, then this is revB. But then you want to return
IBS_CAPS_DEFAULT there.

Thx.

-- 
Regards/Gruss,
    Boris.

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