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

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Date:   Sun, 10 Sep 2017 12:17:10 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Jiri Kosina <jikos@...nel.org>
Cc:     Andy Lutomirski <luto@...nel.org>, 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 Fri, Sep 8, 2017 at 12:59 AM, Jiri Kosina <jikos@...nel.org> wrote:
> 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>
>

Ingo, please do *not* apply this patch yet.  The code is fine, but the
comment is about to become wrong.  I just found a nasty initialization
order issue, and I need to rework a bunch of the way we deal with
PCIDE.