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

[Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>]

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@xxxxxxxxxxxxxxx
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> ---
> 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