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Message-ID: <20240422172055.GAZiacdxkQU0XAbybW@fat_crate.local>
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.
https://people.kernel.org/tglx/notes-about-netiquette
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