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

From: Ingo Molnar
Date: Mon Sep 11 2017 - 01:48:32 EST

* Andy Lutomirski <luto@xxxxxxxxxx> wrote:

> On Fri, Sep 8, 2017 at 12:59 AM, Jiri Kosina <jikos@xxxxxxxxxx> 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@xxxxxxxxxx>
> >> Fixes: 660da7c9228f ("x86/mm: Enable CR4.PCIDE on supported systems")
> >> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
> >
> > Tested-by: Jiri Kosina <jkosina@xxxxxxx>
> >
> 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

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

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.