linux-kernel - Re: [tip: perf/urgent] x86/cpufeatures: Add dedicated feature word for CPUID leaf 0x80000022[EAX]

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Date: Tue, 19 Mar 2024 14:05:31 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Borislav Petkov <bp@...en8.de>
Cc: Sandipan Das <sandipan.das@....com>, linux-kernel@...r.kernel.org,
	linux-tip-commits@...r.kernel.org, x86@...nel.org
Subject: Re: [tip: perf/urgent] x86/cpufeatures: Add dedicated feature word
 for CPUID leaf 0x80000022[EAX]


* Borislav Petkov <bp@...en8.de> wrote:

> On Tue, Mar 19, 2024 at 10:36:01AM -0000, tip-bot2 for Sandipan Das wrote:
> > The following commit has been merged into the perf/urgent branch of tip:
> > 
> > Commit-ID:     f0a22ea644717fa21698a32d342fcd307e53a935
> > Gitweb:        https://git.kernel.org/tip/f0a22ea644717fa21698a32d342fcd307e53a935
> > Author:        Sandipan Das <sandipan.das@....com>
> > AuthorDate:    Tue, 19 Mar 2024 13:48:16 +05:30
> > Committer:     Ingo Molnar <mingo@...nel.org>
> > CommitterDate: Tue, 19 Mar 2024 11:23:47 +01:00
> > 
> > x86/cpufeatures: Add dedicated feature word for CPUID leaf 0x80000022[EAX]
> > 
> > Move the existing scattered performance monitoring related feature bits
> > from CPUID leaf 0x80000022[EAX] into a dedicated word since additional
> > bits will be defined from the same leaf in the future. This includes
> > X86_FEATURE_PERFMON_V2 and X86_FEATURE_AMD_LBR_V2.
> 
> That CPUID leaf has a whopping three bits defined and the rest is
> reserved. You should do a dedicated leaf when we use at least 50% of the
> bits in the leaf. But not like this.
> 
> Please do simply another synthetic leaf and put your bits there.

Yeah, that was my original suggestion too, so fully agreed - I got 
distracted by the cleanup factor, but it's not worth spreading out the 
cpuinfo.x86_capability[] bits unnecessarily.

Thanks,

	Ingo