Subject: Re: [PATCH 2/2] powerpc/mm/radix: Synchronize updates to the process table

From: Benjamin Herrenschmidt <[email-masked]>
Date: Mon, 10 Jul 2017 16:44:42 +1000

On Mon, 2017-07-10 at 14:40 +1000, Nicholas Piggin wrote:
> On Fri, 07 Jul 2017 16:12:16 -0500
> Benjamin Herrenschmidt <[email-masked]> wrote:
> 
> > When writing to the process table, we need to ensure the store is
> > visible to a subsequent access by the MMU. We assume we never have
> > the PID active while doing the update, so a ptesync/isync pair
> > should hopefully be a big enough hammer for our purpose.
> > 
> 
> Do we need this if it's going from invalid->valid?

No. While there is no valid bit in radix, I checked with HW and they
will not cache an entry that has an invalid RTS field. We should ensure
this gets architected for future impl. though.

> 
> > Signed-off-by: Benjamin Herrenschmidt <[email-masked]>
> > ---
> > 
> > Note: Architecturally, we also need to use a tlbie(l) with RIC=2
> > to flush the process table cache. However this is (very) expensive
> > and we know that POWER9 will invalidate its cache when hitting the
> > mtpid instruction.
> > 
> > To be safe, we should add the tlbie for any ARCH300 processor we
> > don't know about though. (Aneesh, Nick do we need a ftr bit ?)
> 
> Good question, I'm not sure. Aside from this particular thing, it
> seems like a good idea in general to add implementation specific
> tests into the ftr framework.
> 
> We could add the PVR into it so we don't have to pollute FTR bits.
> The POWER9_DD1 bit for example could just be a PVR mask and cmp.

Reading the PVR isn't necessarily cheap though, we may want to cache
it.
> 
> > 
> >  arch/powerpc/mm/mmu_context_book3s64.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> > 
> > diff --git a/arch/powerpc/mm/mmu_context_book3s64.c b/arch/powerpc/mm/mmu_context_book3s64.c
> > index 9404b5e..e3e2803 100644
> > --- a/arch/powerpc/mm/mmu_context_book3s64.c
> > +++ b/arch/powerpc/mm/mmu_context_book3s64.c
> > @@ -138,6 +138,14 @@ static int radix__init_new_context(struct mm_struct *mm)
> >  	rts_field = radix__get_tree_size();
> >  	process_tb[index].prtb0 = cpu_to_be64(rts_field | __pa(mm->pgd) | RADIX_PGD_INDEX_SIZE);
> >  
> > +	/*
> > +	 * Order the above store with subsequent update of the PID
> > +	 * register (at which point HW can start loading/caching
> > +	 * the entry) and the corresponding load by the MMU from
> > +	 * the L2 cache.
> > +	 */
> > +	asm volatile("ptesync;isync" : : : "memory");
> > +
> >  	mm->context.npu_context = NULL;
> >  
> >  	return index;
> >