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

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Message-ID: <YwC3Wmj2Fq11EWVg@kroah.com>
Date:   Sat, 20 Aug 2022 12:28:42 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
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>, Borislav Petkov <bp@...en8.de>,
        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 Sat, Aug 20, 2022 at 11:25:24AM +0200, Juergen Gross wrote:
> When booting or resuming the system MTRR state is saved on the boot
> processor and then this state is loaded into MTRRs of all other cpus.
> During update of the MTRRs the MTRR mechanism needs to be disabled by
> writing the related MSR. The old contents of this MSR are saved in a
> set of static variables and later those static variables are used to
> restore the MSR.
> 
> In case the MSR contents need to be modified on a cpu due to the MSR
> not having been initialized properly by the BIOS, the related update
> function is modifying the static variables accordingly.
> 
> Unfortunately the MTRR state update is usually running on all cpus
> at the same time, so using just one set of static variables for all
> cpus is racy in case the MSR contents differ across cpus.
> 
> Fix that by using percpu variables for saving the MSR contents.
> 
> Cc: stable@...r.kernel.org
> Signed-off-by: Juergen Gross <jgross@...e.com>
> ---
> I thought adding a "Fixes:" tag for the kernel's initial git commit
> would maybe be entertaining, but without being really helpful.

So that means I will just do a "best guess" as to how far to backport
things.  Hopefully I guess well...

thanks,

greg k-h