Re: [PATCH] x86 / hibernate: Use hlt_play_dead() when resuming from hibernation

[Pavel Machek]

On Wed 2016-07-13 14:01:52, Rafael J. Wysocki wrote:
> On Wed, Jul 13, 2016 at 11:56 AM, Pavel Machek <pavel@xxxxxx> wrote:
> > On Sun 2016-07-10 03:49:25, Rafael J. Wysocki wrote:
> >> From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> >>
> >> On Intel hardware, native_play_dead() uses mwait_play_dead() by
> >> default and only falls back to the other methods if that fails.
> >> That also happens during resume from hibernation, when the restore
> >> (boot) kernel runs disable_nonboot_cpus() to take all of the CPUs
> >> except for the boot one offline.
> >>
> >> However, that is problematic, because the address passed to
> >> __monitor() in mwait_play_dead() is likely to be written to in the
> >> last phase of hibernate image restoration and that causes the "dead"
> >> CPU to start executing instructions again.  Unfortunately, the page
> >> containing the address in that CPU's instruction pointer may not be
> >> valid any more at that point.
> >>
> >> First, that page may have been overwritten with image kernel memory
> >> contents already, so the instructions the CPU attempts to execute may
> >> simply be invalid.  Second, the page tables previously used by that
> >> CPU may have been overwritten by image kernel memory contents, so the
> >> address in its instruction pointer is impossible to resolve then.
> >>
> >> A report from Varun Koyyalagunta and investigation carried out by
> >> Chen Yu show that the latter sometimes happens in practice.
> >>
> >> To prevent it from happening, modify native_play_dead() to make
> >> it use hlt_play_dead() instead of mwait_play_dead() during resume
> >> from hibernation which avoids the inadvertent "revivals" of "dead"
> >> CPUs.
> >>
> >> A slightly unpleasant consequence of this change is that if the
> >> system is hibernated with one or more CPUs offline, it will generally
> >> draw more power after resume than it did before hibernation, because
> >> the physical state entered by CPUs via hlt_play_dead() is higher-power
> >> than the mwait_play_dead() one in the majority of cases.  It is
> >> possible to work around this, but it is unclear how much of a problem
> >> that's going to be in practice, so the workaround will be implemented
> >> later if it turns out to be necessary.
> >>
> >> Link: https://bugzilla.kernel.org/show_bug.cgi?id=106371
> >> Reported-by: Varun Koyyalagunta <cpudebug@xxxxxxxxxxxx>
> >> Original-by: Chen Yu <yu.c.chen@xxxxxxxxx>
> >> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> >
> > I notice that it changes even i386, where it should not be
> > neccessary. But we probably should switch i386 to support similar to
> > x86-64 one day (and I have patches) so no problem there.
> >
> > But I wonder if simpler solution is to place the mwait semaphore into
> > known address? (Nosave region comes to mind?)
> 
> It might work, but it wouldn't be simpler.
> 
> First off, we'd need to monitor a separate cache line for each CPU
> (see the message from Chen Yu) and it'd be a pain to guarantee that.
> Second, CPUs may be woken up from MWAIT for other reasons, so that
> needs to be taken into account too.
> 
> In principle, we might set up a MONITOR?MWAIT "play dead" loop in a
> safe page and make the "dead" CPUs jump to it during image restore,
> but then the image kernel (after getting control back) would need to
> migrate them away from there again, so doing the "halt" thing is *way*
> simpler than that.

Ok, it looks you have the best solution. Thanks...
									Pavel
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