linux-kernel - Re: [PATCH v2 01/10] x86/mtrr: fix MTRR fixup on APs

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Message-ID: <YwM+GPu8hFowl2R7@zn.tnic>
Date:   Mon, 22 Aug 2022 10:28:08 +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>, stable@...r.kernel.org
Subject: Re: [PATCH v2 01/10] x86/mtrr: fix MTRR fixup on APs

On Mon, Aug 22, 2022 at 07:17:40AM +0200, Juergen Gross wrote:
> And then there is mtrr_state_warn() in arch/x86/kernel/cpu/mtrr/generic.c
> which has a comment saying:
> 
> /* Some BIOS's are messed up and don't set all MTRRs the same! */

That thing also says:

        pr_info("mtrr: probably your BIOS does not setup all CPUs.\n");
        pr_info("mtrr: corrected configuration.\n");

because it'll go and force on all CPUs the MTRR state it read from the
BSP in mtrr_bp_init->get_mtrr_state.

> Yes, the chances are slim to hit such a box,

Well, my workstation says:

$ dmesg | grep -i mtrr
[    0.391514] mtrr: your CPUs had inconsistent variable MTRR settings
[    0.395199] mtrr: probably your BIOS does not setup all CPUs.
[    0.399199] mtrr: corrected configuration.

but that's the variable MTRRs.

> but your reasoning suggests I should remove the related code?

My reasoning says you should not do anything at all here - works as
advertized. :-)

-- 
Regards/Gruss,
    Boris.

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