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

[Andy Lutomirski <luto@amacapital.net>]

> On May 31, 2019, at 2:05 PM, Jiri Kosina <jikos@kernel.org> 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 isn’t all that common and the window where it matters is short.  Or maybe the SDM is misleading.