From: Paolo Bonzini <pbonzini@redhat.com> To: guangrong.xiao@gmail.com, mtosatti@redhat.com, avi.kivity@gmail.com, rkrcmar@redhat.com Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, qemu-devel@nongnu.org, Xiao Guangrong <xiaoguangrong@tencent.com> Subject: Re: [PATCH 0/7] KVM: MMU: fast write protect Date: Wed, 3 May 2017 14:28:16 +0200 [thread overview] Message-ID: <e186ce8d-52fc-3eca-1343-1a2f18331a76@redhat.com> (raw) In-Reply-To: <20170503105224.19049-1-xiaoguangrong@tencent.com> So if I understand correctly this relies on userspace doing: 1) KVM_GET_DIRTY_LOG without write protect 2) KVM_WRITE_PROTECT_ALL_MEM <only look now at the dirty log snapshot> Writes may happen between 1 and 2; they are not represented in the live dirty bitmap but it's okay because they are in the snapshot and will only be used after 2. This is similar to what the dirty page ring buffer patches do; in fact, the KVM_WRITE_PROTECT_ALL_MEM ioctl is very similar to KVM_RESET_DIRTY_PAGES in those patches. On 03/05/2017 12:52, guangrong.xiao@gmail.com wrote: > Comparing with the ordinary algorithm which > write protects last level sptes based on the rmap one by one, > it just simply updates the generation number to ask all vCPUs > to reload its root page table, particularly, it can be done out > of mmu-lock, so that it does not hurt vMMU's parallel. This is clever. For processors that have PML, write protecting is only done on large pages and only for splitting purposes; not for dirty page tracking process at 4k granularity. In this case, I think that you should do nothing in the new write-protect-all ioctl? Also, I wonder how the alternative write protection mechanism would affect performance of the dirty page ring buffer patches. You would do the write protection of all memory at the end of kvm_vm_ioctl_reset_dirty_pages. You wouldn't even need a separate ioctl, which is nice. On the other hand, checkpoints would be more frequent and most pages would be write-protected, so it would be more expensive to rebuild the shadow page tables... Thanks, Paolo > @@ -490,6 +511,7 @@ static int kvm_physical_sync_dirty_bitmap(KVMMemoryListener *kml, > memset(d.dirty_bitmap, 0, allocated_size); > > d.slot = mem->slot | (kml->as_id << 16); > + d.flags = kvm_write_protect_all ? KVM_DIRTY_LOG_WITHOUT_WRITE_PROTECT : 0; > if (kvm_vm_ioctl(s, KVM_GET_DIRTY_LOG, &d) == -1) { > DPRINTF("ioctl failed %d\n", errno); > ret = -1; How would this work when kvm_physical_sync_dirty_bitmap is called from memory_region_sync_dirty_bitmap rather than memory_region_global_dirty_log_sync? Thanks, Paolo
WARNING: multiple messages have this Message-ID (diff)
From: Paolo Bonzini <pbonzini@redhat.com> To: guangrong.xiao@gmail.com, mtosatti@redhat.com, avi.kivity@gmail.com, rkrcmar@redhat.com Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, qemu-devel@nongnu.org, Xiao Guangrong <xiaoguangrong@tencent.com> Subject: Re: [Qemu-devel] [PATCH 0/7] KVM: MMU: fast write protect Date: Wed, 3 May 2017 14:28:16 +0200 [thread overview] Message-ID: <e186ce8d-52fc-3eca-1343-1a2f18331a76@redhat.com> (raw) In-Reply-To: <20170503105224.19049-1-xiaoguangrong@tencent.com> So if I understand correctly this relies on userspace doing: 1) KVM_GET_DIRTY_LOG without write protect 2) KVM_WRITE_PROTECT_ALL_MEM <only look now at the dirty log snapshot> Writes may happen between 1 and 2; they are not represented in the live dirty bitmap but it's okay because they are in the snapshot and will only be used after 2. This is similar to what the dirty page ring buffer patches do; in fact, the KVM_WRITE_PROTECT_ALL_MEM ioctl is very similar to KVM_RESET_DIRTY_PAGES in those patches. On 03/05/2017 12:52, guangrong.xiao@gmail.com wrote: > Comparing with the ordinary algorithm which > write protects last level sptes based on the rmap one by one, > it just simply updates the generation number to ask all vCPUs > to reload its root page table, particularly, it can be done out > of mmu-lock, so that it does not hurt vMMU's parallel. This is clever. For processors that have PML, write protecting is only done on large pages and only for splitting purposes; not for dirty page tracking process at 4k granularity. In this case, I think that you should do nothing in the new write-protect-all ioctl? Also, I wonder how the alternative write protection mechanism would affect performance of the dirty page ring buffer patches. You would do the write protection of all memory at the end of kvm_vm_ioctl_reset_dirty_pages. You wouldn't even need a separate ioctl, which is nice. On the other hand, checkpoints would be more frequent and most pages would be write-protected, so it would be more expensive to rebuild the shadow page tables... Thanks, Paolo > @@ -490,6 +511,7 @@ static int kvm_physical_sync_dirty_bitmap(KVMMemoryListener *kml, > memset(d.dirty_bitmap, 0, allocated_size); > > d.slot = mem->slot | (kml->as_id << 16); > + d.flags = kvm_write_protect_all ? KVM_DIRTY_LOG_WITHOUT_WRITE_PROTECT : 0; > if (kvm_vm_ioctl(s, KVM_GET_DIRTY_LOG, &d) == -1) { > DPRINTF("ioctl failed %d\n", errno); > ret = -1; How would this work when kvm_physical_sync_dirty_bitmap is called from memory_region_sync_dirty_bitmap rather than memory_region_global_dirty_log_sync? Thanks, Paolo
next prev parent reply other threads:[~2017-05-03 12:28 UTC|newest] Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top 2017-05-03 10:52 [PATCH 0/7] KVM: MMU: fast write protect guangrong.xiao 2017-05-03 10:52 ` [Qemu-devel] " guangrong.xiao 2017-05-03 10:52 ` [PATCH 1/7] KVM: MMU: correct the behavior of mmu_spte_update_no_track guangrong.xiao 2017-05-03 10:52 ` [Qemu-devel] " guangrong.xiao 2017-05-03 10:52 ` [PATCH 2/7] KVM: MMU: introduce possible_writable_spte_bitmap guangrong.xiao 2017-05-03 10:52 ` [Qemu-devel] " guangrong.xiao 2017-05-03 10:52 ` [PATCH 3/7] KVM: MMU: introduce kvm_mmu_write_protect_all_pages guangrong.xiao 2017-05-03 10:52 ` [Qemu-devel] " guangrong.xiao 2017-05-03 10:52 ` [PATCH 4/7] KVM: MMU: enable KVM_WRITE_PROTECT_ALL_MEM guangrong.xiao 2017-05-03 10:52 ` [Qemu-devel] " guangrong.xiao 2017-05-03 10:52 ` [PATCH 5/7] KVM: MMU: allow dirty log without write protect guangrong.xiao 2017-05-03 10:52 ` [Qemu-devel] " guangrong.xiao 2017-05-03 10:52 ` [PATCH 6/7] KVM: MMU: clarify fast_pf_fix_direct_spte guangrong.xiao 2017-05-03 10:52 ` [Qemu-devel] " guangrong.xiao 2017-05-03 10:52 ` [PATCH 7/7] KVM: MMU: stop using mmu_spte_get_lockless under mmu-lock guangrong.xiao 2017-05-03 10:52 ` [Qemu-devel] " guangrong.xiao 2017-05-03 12:28 ` Paolo Bonzini [this message] 2017-05-03 12:28 ` [Qemu-devel] [PATCH 0/7] KVM: MMU: fast write protect Paolo Bonzini 2017-05-03 14:50 ` Xiao Guangrong 2017-05-03 14:50 ` [Qemu-devel] " Xiao Guangrong 2017-05-03 14:57 ` Paolo Bonzini 2017-05-03 14:57 ` [Qemu-devel] " Paolo Bonzini 2017-05-04 3:36 ` Xiao Guangrong 2017-05-04 3:36 ` [Qemu-devel] " Xiao Guangrong 2017-05-04 7:06 ` Paolo Bonzini 2017-05-04 7:06 ` [Qemu-devel] " Paolo Bonzini 2017-05-23 2:23 ` Xiao Guangrong 2017-05-23 2:23 ` [Qemu-devel] " Xiao Guangrong 2017-05-29 16:48 ` Paolo Bonzini 2017-05-29 16:48 ` [Qemu-devel] " Paolo Bonzini 2017-06-09 3:19 ` Xiao Guangrong 2017-06-09 3:19 ` [Qemu-devel] " Xiao Guangrong 2017-06-05 7:36 ` Jay Zhou 2017-06-05 7:36 ` Jay Zhou 2017-06-06 2:56 ` Xiao Guangrong
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