Re: [PATCH v4] x86/power: Fix 'nosmt' vs. hibernation triple fault during resume

From: Andy Lutomirski
Date: Fri May 31 2019 - 17:26:13 EST

> On May 31, 2019, at 2:05 PM, Jiri Kosina <jikos@xxxxxxxxxx> wrote:
>> On Fri, 31 May 2019, Andy Lutomirski wrote:
>> The Intel SDM Vol 3 34.10 says:
>> If the HLT instruction is restarted, the processor will generate a
>> memory access to fetch the HLT instruction (if it is
>> not in the internal cache), and execute a HLT bus transaction. This
>> behavior results in multiple HLT bus transactions
>> for the same HLT instruction.
> Which basically means that both hibernation and kexec have been broken in
> this respect for gazillions of years, and seems like noone noticed. Makes
> one wonder what the reason for that might be.
> Either SDM is not precise and the refetch actually never happens for real
> (or is always in these cases satisfied from I$ perhaps?), or ... ?
> So my patch basically puts things back where they have been for ages
> (while mwait is obviously much worse, as that gets woken up by the write
> to the monitored address, which inevitably does happen during resume), but
> seems like SDM is suggesting that we've been in a grey zone wrt RSM at
> least for all those ages.
> So perhaps we really should ditch resume_play_dead() altogether
> eventually, and replace it with sending INIT IPI around instead (and then
> waking the CPUs properly via INIT INIT START). I'd still like to do that
> for 5.3 though, as that'd be slightly bigger surgery, and conservatively
> put things basically back to state they have been up to now for 5.2.

Seems reasonable to me. I would guess that it mostly works because SMI isnât all that common and the window where it matters is short. Or maybe the SDM is misleading.