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

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Date:   Tue, 30 May 2017 13:59:41 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     Dominik Brodowski <linux@...inikbrodowski.net>
cc:     Bernhard Held <berny156@....de>, Andy Lutomirski <luto@...nel.org>,
        Toshi Kani <toshi.kani@...com>, Borislav Petkov <bp@...en8.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Brian Gerst <brgerst@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        "Luis R. Rodriguez" <mcgrof@...e.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [tip:x86/urgent] x86/PAT: Fix Xorg regression on CPUs that don't
 support PAT



On Tue, 30 May 2017, Dominik Brodowski wrote:

> Same boot problem here (Intel(R) Core(TM) i5-5200U CPU on a Dell XPS 13),
> git-bisected to the same patch...
> 
> On Mon, May 29, 2017 at 06:50:57PM -0400, Mikulas Patocka wrote:
> > Please do the following three tests and test if the kernel boots.
> > 
> > 1. use the PAT patch and revert the change to the function pat_enabled()
> > - i.e. change it to the original:
> > bool pat_enabled(void)
> > {
> > 	return !!__pat_enabled;
> > }
> 
> No joy.
> 
> > 2. use the PAT patch and revert the change to the function pat_ap_init
> > - i.e. change it to the original:
> > static void pat_ap_init(u64 pat)
> > {
> > 	if (!boot_cpu_has(X86_FEATURE_PAT)) {
> 
> Joy.

It is interesting - does it mean that the boot cpu does have PAT and the 
secondary CPUs don't? Please send /proc/cpuinfo with all the cores active.

This part of the patch is not required anyway, so I will resubmit the 
patch with this part disabled (and with an added call to 
init_cache_modes() as Andy suggested).

Mikulas

> > 3. use the full PAT patch and apply the below patch on the top of it.
> 
> No joy.
> 
> 
> Best,
> 	Dominik
>