From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753080AbcFOCoZ (ORCPT ); Tue, 14 Jun 2016 22:44:25 -0400 Received: from mail-pf0-f196.google.com ([209.85.192.196]:34418 "EHLO mail-pf0-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752878AbcFOCoX convert rfc822-to-8bit (ORCPT ); Tue, 14 Jun 2016 22:44:23 -0400 Content-Type: text/plain; charset=us-ascii Mime-Version: 1.0 (Mac OS X Mail 9.3 \(3124\)) Subject: Re: [PATCH] Linux VM workaround for Knights Landing A/D leak From: Nadav Amit In-Reply-To: Date: Tue, 14 Jun 2016 19:44:21 -0700 Cc: Dave Hansen , Lukasz Anaczkowski , LKML , "linux-mm@kvack.org" , Thomas Gleixner , Ingo Molnar , Andi Kleen , "Kirill A. Shutemov" , Michal Hocko , Andrew Morton , "H. Peter Anvin" , harish.srinivasappa@intel.com, lukasz.odzioba@intel.com Content-Transfer-Encoding: 8BIT Message-Id: <2E7DAAC8-7EB5-4F79-BDC5-55C8169F4F78@gmail.com> References: <1465919919-2093-1-git-send-email-lukasz.anaczkowski@intel.com> <7FB15233-B347-4A87-9506-A9E10D331292@gmail.com> <57603C61.5000408@linux.intel.com> <2471A3E8-FF69-4720-A3BF-BDC6094A6A70@gmail.com> <5760792D.90000@linux.intel.com> <3878B27E-756E-4086-93BE-B4799A70570F@gmail.com> To: Andy Lutomirski X-Mailer: Apple Mail (2.3124) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Andy Lutomirski wrote: > On Tue, Jun 14, 2016 at 7:35 PM, Nadav Amit wrote: >> Andy Lutomirski wrote: >> >>> On Tue, Jun 14, 2016 at 2:37 PM, Dave Hansen >>> wrote: >>>> On 06/14/2016 01:16 PM, Nadav Amit wrote: >>>>> Dave Hansen wrote: >>>>> >>>>>> On 06/14/2016 09:47 AM, Nadav Amit wrote: >>>>>>> Lukasz Anaczkowski wrote: >>>>>>> >>>>>>>>> From: Andi Kleen >>>>>>>>> +void fix_pte_leak(struct mm_struct *mm, unsigned long addr, pte_t *ptep) >>>>>>>>> +{ >>>>>>> Here there should be a call to smp_mb__after_atomic() to synchronize with >>>>>>> switch_mm. I submitted a similar patch, which is still pending (hint). >>>>>>> >>>>>>>>> + if (cpumask_any_but(mm_cpumask(mm), smp_processor_id()) < nr_cpu_ids) { >>>>>>>>> + trace_tlb_flush(TLB_LOCAL_SHOOTDOWN, TLB_FLUSH_ALL); >>>>>>>>> + flush_tlb_others(mm_cpumask(mm), mm, addr, >>>>>>>>> + addr + PAGE_SIZE); >>>>>>>>> + mb(); >>>>>>>>> + set_pte(ptep, __pte(0)); >>>>>>>>> + } >>>>>>>>> +} >>>>>> >>>>>> Shouldn't that barrier be incorporated in the TLB flush code itself and >>>>>> not every single caller (like this code is)? >>>>>> >>>>>> It is insane to require individual TLB flushers to be concerned with the >>>>>> barriers. >>>>> >>>>> IMHO it is best to use existing flushing interfaces instead of creating >>>>> new ones. >>>> >>>> Yeah, or make these things a _little_ harder to get wrong. That little >>>> snippet above isn't so crazy that we should be depending on open-coded >>>> barriers to get it right. >>>> >>>> Should we just add a barrier to mm_cpumask() itself? That should stop >>>> the race. Or maybe we need a new primitive like: >>>> >>>> /* >>>> * Call this if a full barrier has been executed since the last >>>> * pagetable modification operation. >>>> */ >>>> static int __other_cpus_need_tlb_flush(struct mm_struct *mm) >>>> { >>>> /* cpumask_any_but() returns >= nr_cpu_ids if no cpus set. */ >>>> return cpumask_any_but(mm_cpumask(mm), smp_processor_id()) < >>>> nr_cpu_ids; >>>> } >>>> >>>> >>>> static int other_cpus_need_tlb_flush(struct mm_struct *mm) >>>> { >>>> /* >>>> * Synchronizes with switch_mm. Makes sure that we do not >>>> * observe a bit having been cleared in mm_cpumask() before >>>> * the other processor has seen our pagetable update. See >>>> * switch_mm(). >>>> */ >>>> smp_mb__after_atomic(); >>>> >>>> return __other_cpus_need_tlb_flush(mm) >>>> } >>>> >>>> We should be able to deploy other_cpus_need_tlb_flush() in most of the >>>> cases where we are doing "cpumask_any_but(mm_cpumask(mm), >>>> smp_processor_id()) < nr_cpu_ids". >>> >>> IMO this is a bit nuts. smp_mb__after_atomic() doesn't do anything on >>> x86. And, even if it did, why should the flush code assume that the >>> previous store was atomic? >>> >>> What's the issue being fixed / worked around here? >> >> It does a compiler barrier, which prevents the decision whether a >> remote TLB shootdown is required to be made before the PTE is set. >> >> I agree that PTEs may not be written atomically in certain cases >> (although I am unaware of such cases, except on full-mm flush). > > How about plain set_pte? It's atomic (aligned word-sized write), but > it's not atomic in the _after_atomic sense. Can you point me to a place where set_pte is used before a TLB invalidation/shootdown, excluding this patch and the fullmm case? I am not claiming there is no such case, but I am unaware of such one. PTEs are cleared on SMP using xchg, and similarly the dirty bit is cleared with an atomic operation.