linux-kernel - Re: [PATCH] [RFC] x86/cpu: rework instruction set selection

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Message-Id: <09e26f91-9a62-45c8-b94e-eddb307251f4@app.fastmail.com>
Date: Fri, 25 Apr 2025 18:13:31 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "H. Peter Anvin" <hpa@...or.com>, "Arnd Bergmann" <arnd@...nel.org>,
 "Thomas Gleixner" <tglx@...utronix.de>, "Ingo Molnar" <mingo@...hat.com>,
 "Borislav Petkov" <bp@...en8.de>,
 "Dave Hansen" <dave.hansen@...ux.intel.com>, x86@...nel.org
Cc: "Juergen Gross" <jgross@...e.com>,
 "Boris Ostrovsky" <boris.ostrovsky@...cle.com>,
 "Alexander Usyskin" <alexander.usyskin@...el.com>,
 "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
 Mateusz Jończyk <mat.jonczyk@...pl>,
 "Mike Rapoport" <rppt@...nel.org>, "Ard Biesheuvel" <ardb@...nel.org>,
 "Peter Zijlstra" <peterz@...radead.org>, linux-kernel@...r.kernel.org,
 xen-devel@...ts.xenproject.org
Subject: Re: [PATCH] [RFC] x86/cpu: rework instruction set selection

On Fri, Apr 25, 2025, at 17:34, H. Peter Anvin wrote:
> On April 25, 2025 7:15:15 AM PDT, Arnd Bergmann <arnd@...nel.org> wrote:
>
> I really don't like testing an unrelated feature (CMOV for PAE); 

How about a new symbol with the opposite polarity, e.g. CONFIG_CPU_586?
In that case, X86_HAVE_PAE and X86_CMOV could both depend on that
not being set.

I only picked the X86_CMOV symbol because it already exists in .config
files, but that is not the important bit here.

> furthermore, at least some old hypervisors were known to have
> broken PAE.

I'm not following. What does that have to do with my patch?

     Arnd