From: Peter Xu <peterx@redhat.com> To: Paolo Bonzini <pbonzini@redhat.com> Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Sean Christopherson <sean.j.christopherson@intel.com>, "Dr . David Alan Gilbert" <dgilbert@redhat.com>, Vitaly Kuznetsov <vkuznets@redhat.com> Subject: Re: [PATCH RFC 00/15] KVM: Dirty ring interface Date: Thu, 5 Dec 2019 15:52:18 -0500 Message-ID: <20191205205218.GB7201@xz-x1> (raw) In-Reply-To: <60888f25-2299-2a04-68c2-6eca171a2a18@redhat.com> On Thu, Dec 05, 2019 at 08:59:33PM +0100, Paolo Bonzini wrote: > On 05/12/19 20:30, Peter Xu wrote: > >> Try enabling kvmmmu tracepoints too, it will tell > >> you more of the path that was taken while processing the EPT violation. > > > > These new tracepoints are extremely useful (which I didn't notice > > before). > > Yes, they are! (I forgot to say thanks for teaching me that! :) > > > So here's the final culprit... > > > > void kvm_reset_dirty_gfn(struct kvm *kvm, u32 slot, u64 offset, u64 mask) > > { > > ... > > spin_lock(&kvm->mmu_lock); > > /* FIXME: we should use a single AND operation, but there is no > > * applicable atomic API. > > */ > > while (mask) { > > clear_bit_le(offset + __ffs(mask), memslot->dirty_bitmap); > > mask &= mask - 1; > > } > > > > kvm_arch_mmu_enable_log_dirty_pt_masked(kvm, memslot, offset, mask); > > spin_unlock(&kvm->mmu_lock); > > } > > > > The mask is cleared before reaching > > kvm_arch_mmu_enable_log_dirty_pt_masked().. > > I'm not sure why that results in two vmexits? (clearing before > kvm_arch_mmu_enable_log_dirty_pt_masked is also what > KVM_{GET,CLEAR}_DIRTY_LOG does). Sorry my fault to be not clear on this. The kvm_arch_mmu_enable_log_dirty_pt_masked() only explains why the same page is not written again after the ring-full userspace exit (which triggered the real dirty bit missing), and that's because the write bit is not removed during KVM_RESET_DIRTY_RINGS so the next vmenter will directly write to the previous page without vmexit. The two vmexits is another story - I tracked it is retried because mmu_notifier_seq has changed, hence it goes through this path: if (mmu_notifier_retry(vcpu->kvm, mmu_seq)) goto out_unlock; It's because when try_async_pf(), we will do a writable page fault, which probably triggers both the invalidate_range_end and change_pte notifiers. A reference trace when EPT enabled: kvm_mmu_notifier_change_pte+1 __mmu_notifier_change_pte+82 wp_page_copy+1907 do_wp_page+478 __handle_mm_fault+3395 handle_mm_fault+196 __get_user_pages+681 get_user_pages_unlocked+172 __gfn_to_pfn_memslot+290 try_async_pf+141 tdp_page_fault+326 kvm_mmu_page_fault+115 kvm_arch_vcpu_ioctl_run+2675 kvm_vcpu_ioctl+536 do_vfs_ioctl+1029 ksys_ioctl+94 __x64_sys_ioctl+22 do_syscall_64+91 I'm not sure whether that's ideal, but it makes sense to me. > > > The funny thing is that I did have a few more patches to even skip > > allocate the dirty_bitmap when dirty ring is enabled (hence in that > > tree I removed this while loop too, so that has no such problem). > > However I dropped those patches when I posted the RFC because I don't > > think it's mature, and the selftest didn't complain about that > > either.. Though, I do plan to redo that in v2 if you don't disagree. > > The major question would be whether the dirty_bitmap could still be > > for any use if dirty ring is enabled. > > Userspace may want a dirty bitmap in addition to a list (for example: > list for migration, bitmap for framebuffer update), but it can also do a > pass over the dirty rings in order to update an internal bitmap. > > So I think it make sense to make it either one or the other. Ok, then I'll do. Thanks, -- Peter Xu
next prev parent reply index Thread overview: 123+ messages / expand[flat|nested] mbox.gz Atom feed top 2019-11-29 21:34 Peter Xu 2019-11-29 21:34 ` [PATCH RFC 01/15] KVM: Move running VCPU from ARM to common code Peter Xu 2019-12-03 19:01 ` Sean Christopherson 2019-12-04 9:42 ` Paolo Bonzini 2019-12-09 22:05 ` Peter Xu 2019-11-29 21:34 ` [PATCH RFC 02/15] KVM: Add kvm/vcpu argument to mark_dirty_page_in_slot Peter Xu 2019-12-02 19:32 ` Sean Christopherson 2019-12-02 20:49 ` Peter Xu 2019-11-29 21:34 ` [PATCH RFC 03/15] KVM: Add build-time error check on kvm_run size Peter Xu 2019-12-02 19:30 ` Sean Christopherson 2019-12-02 20:53 ` Peter Xu 2019-12-02 22:19 ` Sean Christopherson 2019-12-02 22:40 ` Peter Xu 2019-12-03 5:50 ` Sean Christopherson 2019-12-03 13:41 ` Paolo Bonzini 2019-12-03 17:04 ` Peter Xu 2019-11-29 21:34 ` [PATCH RFC 04/15] KVM: Implement ring-based dirty memory tracking Peter Xu 2019-12-02 20:10 ` Sean Christopherson 2019-12-02 21:16 ` Peter Xu 2019-12-02 21:50 ` Sean Christopherson 2019-12-02 23:09 ` Peter Xu 2019-12-03 13:48 ` Paolo Bonzini 2019-12-03 18:46 ` Sean Christopherson 2019-12-04 10:05 ` Paolo Bonzini 2019-12-07 0:29 ` Sean Christopherson 2019-12-09 9:37 ` Paolo Bonzini 2019-12-09 21:54 ` Peter Xu 2019-12-10 10:07 ` Paolo Bonzini 2019-12-10 15:52 ` Peter Xu 2019-12-10 17:09 ` Paolo Bonzini 2019-12-15 17:21 ` Peter Xu 2019-12-16 10:08 ` Paolo Bonzini 2019-12-16 18:54 ` Peter Xu 2019-12-17 9:01 ` Paolo Bonzini 2019-12-17 16:24 ` Peter Xu 2019-12-17 16:28 ` Paolo Bonzini 2019-12-18 21:58 ` Peter Xu 2019-12-18 22:24 ` Sean Christopherson 2019-12-18 22:37 ` Paolo Bonzini 2019-12-18 22:49 ` Peter Xu 2019-12-17 2:28 ` Tian, Kevin 2019-12-17 16:18 ` Alex Williamson 2019-12-17 16:30 ` Paolo Bonzini 2019-12-18 0:29 ` Tian, Kevin [not found] ` <AADFC41AFE54684AB9EE6CBC0274A5D19D645E5F@SHSMSX104.ccr.corp.intel.com> 2019-12-17 5:17 ` Tian, Kevin 2019-12-17 5:25 ` Yan Zhao 2019-12-17 16:24 ` Alex Williamson 2019-12-03 19:13 ` Sean Christopherson 2019-12-04 10:14 ` Paolo Bonzini 2019-12-04 14:33 ` Sean Christopherson 2019-12-04 10:38 ` Jason Wang 2019-12-04 11:04 ` Paolo Bonzini 2019-12-04 19:52 ` Peter Xu 2019-12-05 6:51 ` Jason Wang 2019-12-05 12:08 ` Peter Xu 2019-12-05 13:12 ` Jason Wang 2019-12-10 13:25 ` Michael S. Tsirkin 2019-12-10 13:31 ` Paolo Bonzini 2019-12-10 16:02 ` Peter Xu 2019-12-10 21:53 ` Michael S. Tsirkin 2019-12-11 9:05 ` Paolo Bonzini 2019-12-11 13:04 ` Michael S. Tsirkin 2019-12-11 14:54 ` Peter Xu 2019-12-10 21:48 ` Michael S. Tsirkin 2019-12-11 12:53 ` Michael S. Tsirkin 2019-12-11 14:14 ` Paolo Bonzini 2019-12-11 20:59 ` Peter Xu 2019-12-11 22:57 ` Michael S. Tsirkin 2019-12-12 0:08 ` Paolo Bonzini 2019-12-12 7:36 ` Michael S. Tsirkin 2019-12-12 8:12 ` Paolo Bonzini 2019-12-12 10:38 ` Michael S. Tsirkin 2019-12-15 17:33 ` Peter Xu 2019-12-16 9:47 ` Michael S. Tsirkin 2019-12-16 15:07 ` Peter Xu 2019-12-16 15:33 ` Michael S. Tsirkin 2019-12-16 15:47 ` Peter Xu 2019-12-11 17:24 ` Christophe de Dinechin 2019-12-13 20:23 ` Peter Xu 2019-12-14 7:57 ` Paolo Bonzini 2019-12-14 16:26 ` Peter Xu 2019-12-16 9:29 ` Paolo Bonzini 2019-12-16 15:26 ` Peter Xu 2019-12-16 15:31 ` Paolo Bonzini 2019-12-16 15:43 ` Peter Xu 2019-12-17 12:16 ` Christophe de Dinechin 2019-12-17 12:19 ` Paolo Bonzini 2019-12-17 15:38 ` Peter Xu 2019-12-17 16:31 ` Paolo Bonzini 2019-12-17 16:42 ` Peter Xu 2019-12-17 16:48 ` Paolo Bonzini 2019-12-17 19:41 ` Peter Xu 2019-12-18 0:33 ` Paolo Bonzini 2019-12-18 16:32 ` Peter Xu 2019-12-18 16:41 ` Paolo Bonzini 2019-12-20 18:19 ` Peter Xu 2019-11-29 21:34 ` [PATCH RFC 05/15] KVM: Make dirty ring exclusive to dirty bitmap log Peter Xu 2019-11-29 21:34 ` [PATCH RFC 06/15] KVM: Introduce dirty ring wait queue Peter Xu 2019-11-29 21:34 ` [PATCH RFC 07/15] KVM: X86: Implement ring-based dirty memory tracking Peter Xu 2019-11-29 21:34 ` [PATCH RFC 08/15] KVM: selftests: Always clear dirty bitmap after iteration Peter Xu 2019-11-29 21:34 ` [PATCH RFC 09/15] KVM: selftests: Sync uapi/linux/kvm.h to tools/ Peter Xu 2019-11-29 21:35 ` [PATCH RFC 10/15] KVM: selftests: Use a single binary for dirty/clear log test Peter Xu 2019-11-29 21:35 ` [PATCH RFC 11/15] KVM: selftests: Introduce after_vcpu_run hook for dirty " Peter Xu 2019-11-29 21:35 ` [PATCH RFC 12/15] KVM: selftests: Add dirty ring buffer test Peter Xu 2019-11-29 21:35 ` [PATCH RFC 13/15] KVM: selftests: Let dirty_log_test async for dirty ring test Peter Xu 2019-11-29 21:35 ` [PATCH RFC 14/15] KVM: selftests: Add "-c" parameter to dirty log test Peter Xu 2019-11-29 21:35 ` [PATCH RFC 15/15] KVM: selftests: Test dirty ring waitqueue Peter Xu 2019-11-30 8:29 ` [PATCH RFC 00/15] KVM: Dirty ring interface Paolo Bonzini 2019-12-02 2:13 ` Peter Xu 2019-12-03 13:59 ` Paolo Bonzini 2019-12-05 19:30 ` Peter Xu 2019-12-05 19:59 ` Paolo Bonzini 2019-12-05 20:52 ` Peter Xu [this message] 2019-12-02 20:21 ` Sean Christopherson 2019-12-02 20:43 ` Peter Xu 2019-12-04 10:39 ` Jason Wang 2019-12-04 19:33 ` Peter Xu 2019-12-05 6:49 ` Jason Wang 2019-12-11 13:41 ` Christophe de Dinechin 2019-12-11 14:16 ` Paolo Bonzini 2019-12-11 17:15 ` Peter Xu -- strict thread matches above, loose matches on Subject: below -- 2019-11-29 21:33 Peter Xu 2019-11-29 21:32 Peter Xu
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