Re: [tip:x86/urgent] x86/PAT: Fix Xorg regression on CPUs that don't support PAT

[Andy Lutomirski <luto@kernel.org>]

On Sun, May 28, 2017 at 11:18 AM, Bernhard Held <berny156@gmx.de> wrote:
> Hi,
>
> this patch breaks the boot of my kernel. The last message is "Booting
> the kernel.".
>
> My setup might be unusual: I'm running a Xenon E5450 (LGA 771) in a
> Gigbayte G33-DS3R board (LGA 775). The BIOS is patched with the
> microcode of the E5450 and recognizes the CPU.
>
> Please find below the dmesg of a the latest kernel w/o the PAT-patch.
> I'm happy to provide more information or to test patches.

I think this patch is bogus.  pat_enabled() sure looks like it's
supposed to return true if PAT is *enabled*, and these days PAT is
"enabled" even if there's no HW PAT support.  Even if the patch were
somehow correct, it should have been split up into two patches, one to
change pat_enabled() and one to use this_cpu_has().

Ingo, I'd suggest reverting the patch, cc-ing stable on the revert so
-stable knows not to backport it, and starting over with the fix.
>From very brief inspection, the right fix is to make sure that
pat_init(), or at least init_cache_modes(), gets called on the
affected CPUs.

As a future cleanup, I think that pat_enabled() could be deleted
outright and, if needed, replaced by functions like have_memtype_wc()
or similar.  (Do we already have helpers like that?)  Toshi, am I
right?

--Andy