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

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Date:   Mon, 11 Sep 2017 07:48:19 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Jiri Kosina <jikos@...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


* Andy Lutomirski <luto@...nel.org> wrote:

> 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.

Ok, I'll delay everything PCID delayed - once you've gathered it all together 
please send a full series against Linus's latest collecting all the 
fixes/cleanups.

If you find unexpected complications then there will be a point in time where it 
might be better to just disable PCID for this release and re-try in v4.15. As the 
number and complexity of fixes increases so does the risk that we'll introduce 
some last-minute regression.

Thanks,

	Ingo