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

[Nicholas Piggin <npiggin@au1.ibm.com>]

On Fri, 07 Jul 2017 16:12:16 -0500
Benjamin Herrenschmidt <benh@kernel.crashing.org> 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?

> Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> ---
> 
> 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.


> 
>  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;
>