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

[Rafael J. Wysocki]

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.

Thanks,
Rafael