Subject: Re: [PATCH] powerpc/powernv/npu: Remove redundant change_pte() hook

From: Andrea Arcangeli <[email-masked]>
Date: Thu, 31 Jan 2019 12:11:06 -0500

On Thu, Jan 31, 2019 at 06:30:22PM +0800, Peter Xu wrote:
> The change_pte() notifier was designed to use as a quick path to
> update secondary MMU PTEs on write permission changes or PFN changes.
> For KVM, it could reduce the vm-exits when vcpu faults on the pages
> that was touched up by KSM.  It's not used to do cache invalidations,
> for example, if we see the notifier will be called before the real PTE
> update after all (please see set_pte_at_notify that set_pte_at was
> called later).
> 
> All the necessary cache invalidation should all be done in
> invalidate_range() already.
> 
> CC: Benjamin Herrenschmidt <[email-masked]>
> CC: Paul Mackerras <[email-masked]>
> CC: Michael Ellerman <[email-masked]>
> CC: Alistair Popple <[email-masked]>
> CC: Alexey Kardashevskiy <[email-masked]>
> CC: Mark Hairgrove <[email-masked]>
> CC: Balbir Singh <[email-masked]>
> CC: David Gibson <[email-masked]>
> CC: Andrea Arcangeli <[email-masked]>
> CC: Jerome Glisse <[email-masked]>
> CC: Jason Wang <[email-masked]>
> CC: [email-masked]
> CC: [email-masked]
> Signed-off-by: Peter Xu <[email-masked]>
> ---
>  arch/powerpc/platforms/powernv/npu-dma.c | 10 ----------
>  1 file changed, 10 deletions(-)

Reviewed-by: Andrea Arcangeli <[email-masked]>

It doesn't make sense to implement change_pte as an invalidate,
change_pte is not compulsory to implement so if one wants to have
invalidates only, change_pte method shouldn't be implemented in the
first place and the common code will guarantee to invoke the range
invalidates instead.

Currently the whole change_pte optimization is effectively disabled as
noted in past discussions with Jerome (because of the range
invalidates that always surrounds it), so we need to revisit the whole
change_pte logic and decide it to re-enable it or to drop it as a
whole, but in the meantime it's good to cleanup spots like below that
should leave change_pte alone.

There are several examples of mmu_notifiers_ops in the kernel that
don't implement change_pte, in fact it's the majority. Of all mmu
notifier users, only nv_nmmu_notifier_ops, intel_mmuops_change and
kvm_mmu_notifier_ops implements change_pte and as Peter found out by
source review nv_nmmu_notifier_ops, intel_mmuops_change are wrong
about it and should stop implementing it as an invalidate.

In short change_pte is only implemented correctly from KVM which can
really updates the spte and flushes the TLB but the spte update
remains and could avoid a vmexit if we figure out how to re-enable the
optimization safely (the TLB fill after change_pte in KVM EPT/shadow
secondary MMU will be looked up by the CPU in hardware).

If change_pte is implemented, it should update the mapping like KVM
does and not do an invalidate.

Thanks,
Andrea

> 
> diff --git a/arch/powerpc/platforms/powernv/npu-dma.c b/arch/powerpc/platforms/powernv/npu-dma.c
> index 3f58c7dbd581..c003b29d870e 100644
> --- a/arch/powerpc/platforms/powernv/npu-dma.c
> +++ b/arch/powerpc/platforms/powernv/npu-dma.c
> @@ -917,15 +917,6 @@ static void pnv_npu2_mn_release(struct mmu_notifier *mn,
>  	mmio_invalidate(npu_context, 0, ~0UL);
>  }
>  
> -static void pnv_npu2_mn_change_pte(struct mmu_notifier *mn,
> -				struct mm_struct *mm,
> -				unsigned long address,
> -				pte_t pte)
> -{
> -	struct npu_context *npu_context = mn_to_npu_context(mn);
> -	mmio_invalidate(npu_context, address, PAGE_SIZE);
> -}
> -
>  static void pnv_npu2_mn_invalidate_range(struct mmu_notifier *mn,
>  					struct mm_struct *mm,
>  					unsigned long start, unsigned long end)
> @@ -936,7 +927,6 @@ static void pnv_npu2_mn_invalidate_range(struct mmu_notifier *mn,
>  
>  static const struct mmu_notifier_ops nv_nmmu_notifier_ops = {
>  	.release = pnv_npu2_mn_release,
> -	.change_pte = pnv_npu2_mn_change_pte,
>  	.invalidate_range = pnv_npu2_mn_invalidate_range,
>  };
>  
> -- 
> 2.17.1
>