Re: [PATCH] powerpc: Invalidate ERAT on powersave wakeup for POWER9

[Nicholas Piggin <npiggin@gmail.com>]

On Fri, 23 Jun 2017 19:33:23 +1000
Michael Ellerman <mpe@ellerman.id.au> wrote:

> Michael Neuling <mikey@neuling.org> writes:
> 
> > On POWER9 the ERAT may be incorrect on wakeup from some stop states
> > that lose state. This causes random segvs and illegal instructions
> > when these stop states are enabled.  
> 
> Incorrect how?

It can have stale ERAT entries from another idle thread.

> 
> Because with the ERAT flush where you've put it, there's still a good
> amount of code executed prior to the flush isn't there?
> 
> ie. we come in at 0x100, do some of the prolog, do IDLE_TEST which takes
> us to pnv_powersave_wakeup, which then restores state from the paca
> (memory), that returns and then we check KVM ... and then finally we end
> up at pnv_wakeup_loss.

In the case of an HMI, we could call into OPAL as well.

> Or is there some other path? Or is the ERAT incorrect in some specific
> way which means we only need to flush there?

I think we're in real mode until returning from pnv_wakeup_loss so those
ERATs should be the same.

Except KVM, which can go to guest and switch on the MMU. My bad, I
suggested putting it into pnv_wakeup_loss.

Flushing at the start of pnv_powersave_wakeup should be safest. I guess
we can avoid it for non-state-loss wakeups if cr3 is lt.

Thanks,
Nick