linux-kernel - Re: [PATCH v2 02/10] x86/mtrr: remove unused cyrix_set

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Message-ID: <YwdgGFJWTV1YF3n2@zn.tnic>
Date:   Thu, 25 Aug 2022 13:42:16 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Juergen Gross <jgross@...e.com>
Cc:     xen-devel@...ts.xenproject.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2 02/10] x86/mtrr: remove unused cyrix_set_all() function

On Thu, Aug 25, 2022 at 12:41:05PM +0200, Juergen Gross wrote:
> Maybe the alternative reasoning is much faster to understand: if the
> Cyrix set_all() could be called, the AMD and Centaur ones would be callable,
> too.

Right.

> Those being called would result in a NULL deref, so why should we keep
> the Cyrix one?

I know you're eager to remove dead code - I'd love that too. But before
we do that, we need to find out whether some Cyrix hw out there would
not need this.

I know, I know, they should've complained by now ... maybe they have but
we haven't heard about it.

What it most likely looks like is that those machines - a commit from
before git

commit 8fbdcb188e31ac901e216b466b97e90e8b057daa
Author: Dave Jones <davej@...e.de>
Date:   Wed Aug 14 21:14:22 2002 -0700

    [PATCH] Modular x86 MTRR driver.

talks about

+/*
+ * On Cyrix 6x86(MX) and M II the ARR3 is special: it has connection
+ * with the SMM (System Management Mode) mode. So we need the following:
+ * Check whether SMI_LOCK (CCR3 bit 0) is set
+ *   if it is set, write a warning message: ARR3 cannot be changed!
+ *     (it cannot be changed until the next processor reset)

which sounds like old rust. And which no one uses or such machines are
long dead already.

Wikipedia says:

https://en.wikipedia.org/wiki/Cyrix_6x86

"The Cyrix 6x86 is a line of sixth-generation, 32-bit x86
microprocessors designed and released by Cyrix in 1995..."

So I'm thinking removing it would be ok...

-- 
Regards/Gruss,
    Boris.

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