linux-kernel - Re: [PATCH 2/2] x86/hibernate/64: Mask off CR3's PCID bits in the saved CR3

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Message-ID: <nycvar.YFH.7.76.1709080957320.3285@jbgna.fhfr.qr>
Date:   Fri, 8 Sep 2017 09:59:49 +0200 (CEST)
From:   Jiri Kosina <jikos@...nel.org>
To:     Andy Lutomirski <luto@...nel.org>
cc:     X86 ML <x86@...nel.org>, Borislav Petkov <bpetkov@...e.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 2/2] x86/hibernate/64: Mask off CR3's PCID bits in the
 saved CR3

On Thu, 7 Sep 2017, Andy Lutomirski wrote:

> Jiri reported a resume-from-hibernation failure triggered by PCID.
> The root cause appears to be rather odd.  The hibernation asm
> restores a CR3 value that comes from the image header.  If the image
> kernel has PCID on, it's entirely reasonable for this CR3 value to
> have one of the low 12 bits set.  The restore code restores it with
> CR4.PCIDE=0, which means that those low 12 bits are accepted by the
> CPU but are either ignored or interpreted as a caching mode.  This
> is odd, but still works.  We blow up later when the image kernel
> restores CR4, though, since changing CR4.PCIDE with CR3[11:0] != 0
> is illegal.  Boom!
> 
> FWIW, it's entirely unclear to me what's supposed to happen if a PAE
> kernel restores a non-PAE image or vice versa.  Ditto for LA57.

I've just performed 15 hibernation cycles with current Linus' tree 
(5969d1bb3082) with these two patches applied on top of it, and I haven't 
encountered any issue (and the warning in switch_mm_irqs_off() didn't 
trigger either).

> Reported-by: Jiri Kosina <jikos@...nel.org>
> Fixes: 660da7c9228f ("x86/mm: Enable CR4.PCIDE on supported systems")
> Signed-off-by: Andy Lutomirski <luto@...nel.org>

	Tested-by: Jiri Kosina <jkosina@...e.cz>

Thanks!

-- 
Jiri Kosina
SUSE Labs