Messages in this thread Patch in this message | | | Subject | Re: [RFC PATCH 3/4] KVM: arm64: Install the block entry before unmapping the page mappings | From | "wangyanan (Y)" <> | Date | Sun, 28 Feb 2021 19:11:56 +0800 |
| |
On 2021/2/8 19:22, Yanan Wang wrote: > When KVM needs to coalesce the normal page mappings into a block mapping, > we currently invalidate the old table entry first followed by invalidation > of TLB, then unmap the page mappings, and install the block entry at last. > > It will cost a long time to unmap the numerous page mappings, which means > there will be a long period when the table entry can be found invalid. > If other vCPUs access any guest page within the block range and find the > table entry invalid, they will all exit from guest with a translation fault > which is not necessary. And KVM will make efforts to handle these faults, > especially when performing CMOs by block range. > > So let's quickly install the block entry at first to ensure uninterrupted > memory access of the other vCPUs, and then unmap the page mappings after > installation. This will reduce most of the time when the table entry is > invalid, and avoid most of the unnecessary translation faults. BTW: Here show the benefit of this patch alone for reference (testing based on patch1) . This patch aims to speed up the reconstruction of block mappings(especially for 1G blocks) after they have been split, and the following test results represent the significant change. Selftest: https://lore.kernel.org/lkml/20210208090841.333724-1-wangyanan55@huawei.com/
---
hardware platform: HiSilicon Kunpeng920 Server(FWB not supported) host kernel: Linux mainline v5.11-rc6 (with series of https://lore.kernel.org/r/20210114121350.123684-4-wangyanan55@huawei.com applied)
multiple vcpus concurrently access 20G memory. execution time of KVM reconstituting the block mappings after dirty logging.
cmdline: ./kvm_page_table_test -m 4 -t 2 -g 1G -s 20G -v 20 (20 vcpus, 20G memory, block mappings(HUGETLB 1G)) Before patch: KVM_ADJUST_MAPPINGS: 2.881s 2.883s 2.885s 2.879s 2.882s After patch: KVM_ADJUST_MAPPINGS: 0.310s 0.301s 0.312s 0.299s 0.306s *average 89% improvement*
cmdline: ./kvm_page_table_test -m 4 -t 2 -g 1G -s 20G -v 40 (40 vcpus, 20G memory, block mappings(HUGETLB 1G)) Before patch: KVM_ADJUST_MAPPINGS: 2.954s 2.955s 2.949s 2.951s 2.953s After patch: KVM_ADJUST_MAPPINGS: 0.381s 0.366s 0.381s 0.380s 0.378s *average 87% improvement*
cmdline: ./kvm_page_table_test -m 4 -t 2 -g 1G -s 20G -v 60 (60 vcpus, 20G memory, block mappings(HUGETLB 1G)) Before patch: KVM_ADJUST_MAPPINGS: 3.118s 3.112s 3.130s 3.128s 3.119s After patch: KVM_ADJUST_MAPPINGS: 0.524s 0.534s 0.536s 0.525s 0.539s *average 83% improvement*
---
Thanks,
Yanan > > Signed-off-by: Yanan Wang <wangyanan55@huawei.com> > --- > arch/arm64/kvm/hyp/pgtable.c | 26 ++++++++++++-------------- > 1 file changed, 12 insertions(+), 14 deletions(-) > > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c > index 78a560446f80..308c36b9cd21 100644 > --- a/arch/arm64/kvm/hyp/pgtable.c > +++ b/arch/arm64/kvm/hyp/pgtable.c > @@ -434,6 +434,7 @@ struct stage2_map_data { > kvm_pte_t attr; > > kvm_pte_t *anchor; > + kvm_pte_t *follow; > > struct kvm_s2_mmu *mmu; > struct kvm_mmu_memory_cache *memcache; > @@ -553,15 +554,14 @@ static int stage2_map_walk_table_pre(u64 addr, u64 end, u32 level, > if (!kvm_block_mapping_supported(addr, end, data->phys, level)) > return 0; > > - kvm_set_invalid_pte(ptep); > - > /* > - * Invalidate the whole stage-2, as we may have numerous leaf > - * entries below us which would otherwise need invalidating > - * individually. > + * If we need to coalesce existing table entries into a block here, > + * then install the block entry first and the sub-level page mappings > + * will be unmapped later. > */ > - kvm_call_hyp(__kvm_tlb_flush_vmid, data->mmu); > data->anchor = ptep; > + data->follow = kvm_pte_follow(*ptep); > + stage2_coalesce_tables_into_block(addr, level, ptep, data); > return 0; > } > > @@ -614,20 +614,18 @@ static int stage2_map_walk_table_post(u64 addr, u64 end, u32 level, > kvm_pte_t *ptep, > struct stage2_map_data *data) > { > - int ret = 0; > - > if (!data->anchor) > return 0; > > - free_page((unsigned long)kvm_pte_follow(*ptep)); > - put_page(virt_to_page(ptep)); > - > - if (data->anchor == ptep) { > + if (data->anchor != ptep) { > + free_page((unsigned long)kvm_pte_follow(*ptep)); > + put_page(virt_to_page(ptep)); > + } else { > + free_page((unsigned long)data->follow); > data->anchor = NULL; > - ret = stage2_map_walk_leaf(addr, end, level, ptep, data); > } > > - return ret; > + return 0; > } > > /*
| |