On Thursday, 31 January 2019 12:11:06 PM AEDT Andrea Arcangeli wrote: > 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).
Thanks for the fixup. I didn't realise that invalidate_range() always gets called but I now see that is the case so this change looks good to me as well. Reviewed-by: Alistair Popple <alist...@popple.id.au> > > All the necessary cache invalidation should all be done in > > invalidate_range() already. > > > > CC: Benjamin Herrenschmidt <b...@kernel.crashing.org> > > CC: Paul Mackerras <pau...@samba.org> > > CC: Michael Ellerman <m...@ellerman.id.au> > > CC: Alistair Popple <alist...@popple.id.au> > > CC: Alexey Kardashevskiy <a...@ozlabs.ru> > > CC: Mark Hairgrove <mhairgr...@nvidia.com> > > CC: Balbir Singh <bsinghar...@gmail.com> > > CC: David Gibson <da...@gibson.dropbear.id.au> > > CC: Andrea Arcangeli <aarca...@redhat.com> > > CC: Jerome Glisse <jgli...@redhat.com> > > CC: Jason Wang <jasow...@redhat.com> > > CC: linuxppc-dev@lists.ozlabs.org > > CC: linux-ker...@vger.kernel.org > > Signed-off-by: Peter Xu <pet...@redhat.com> > > --- > > > > arch/powerpc/platforms/powernv/npu-dma.c | 10 ---------- > > 1 file changed, 10 deletions(-) > > Reviewed-by: Andrea Arcangeli <aarca...@redhat.com> > > 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, > > > > };