On 25/09/20 23:22, Ben Gardon wrote: > Over the years, the needs for KVM's x86 MMU have grown from running small > guests to live migrating multi-terabyte VMs with hundreds of vCPUs. Where > we previously depended on shadow paging to run all guests, we now have > two dimensional paging (TDP). This patch set introduces a new > implementation of much of the KVM MMU, optimized for running guests with > TDP. We have re-implemented many of the MMU functions to take advantage of > the relative simplicity of TDP and eliminate the need for an rmap. > Building on this simplified implementation, a future patch set will change > the synchronization model for this "TDP MMU" to enable more parallelism > than the monolithic MMU lock. A TDP MMU is currently in use at Google > and has given us the performance necessary to live migrate our 416 vCPU, > 12TiB m2-ultramem-416 VMs. > > This work was motivated by the need to handle page faults in parallel for > very large VMs. When VMs have hundreds of vCPUs and terabytes of memory, > KVM's MMU lock suffers extreme contention, resulting in soft-lockups and > long latency on guest page faults. This contention can be easily seen > running the KVM selftests demand_paging_test with a couple hundred vCPUs. > Over a 1 second profile of the demand_paging_test, with 416 vCPUs and 4G > per vCPU, 98% of the time was spent waiting for the MMU lock. At Google, > the TDP MMU reduced the test duration by 89% and the execution was > dominated by get_user_pages and the user fault FD ioctl instead of the > MMU lock. > > This series is the first of two. In this series we add a basic > implementation of the TDP MMU. In the next series we will improve the > performance of the TDP MMU and allow it to execute MMU operations > in parallel. > > The overall purpose of the KVM MMU is to program paging structures > (CR3/EPT/NPT) to encode the mapping of guest addresses to host physical > addresses (HPA), and to provide utilities for other KVM features, for > example dirty logging. The definition of the L1 guest physical address > (GPA) to HPA mapping comes in two parts: KVM's memslots map GPA to HVA, > and the kernel MM/x86 host page tables map HVA -> HPA. Without TDP, the > MMU must program the x86 page tables to encode the full translation of > guest virtual addresses (GVA) to HPA. This requires "shadowing" the > guest's page tables to create a composite x86 paging structure. This > solution is complicated, requires separate paging structures for each > guest CR3, and requires emulating guest page table changes. The TDP case > is much simpler. In this case, KVM lets the guest control CR3 and programs > the EPT/NPT paging structures with the GPA -> HPA mapping. The guest has > no way to change this mapping and only one version of the paging structure > is needed per L1 paging mode. In this case the paging mode is some > combination of the number of levels in the paging structure, the address > space (normal execution or system management mode, on x86), and other > attributes. Most VMs only ever use 1 paging mode and so only ever need one > TDP structure. > > This series implements a "TDP MMU" through alternative implementations of > MMU functions for running L1 guests with TDP. The TDP MMU falls back to > the existing shadow paging implementation when TDP is not available, and > interoperates with the existing shadow paging implementation for nesting. > The use of the TDP MMU can be controlled by a module parameter which is > snapshot on VM creation and follows the life of the VM. This snapshot > is used in many functions to decide whether or not to use TDP MMU handlers > for a given operation. > > This series can also be viewed in Gerrit here: > https://linux-review.googlesource.com/c/virt/kvm/kvm/+/2538 > (Thanks to Dmitry Vyukov <dvyukov@xxxxxxxxxx> for setting up the > Gerrit instance) > > Ben Gardon (22): > kvm: mmu: Separate making SPTEs from set_spte > kvm: mmu: Introduce tdp_iter > kvm: mmu: Init / Uninit the TDP MMU > kvm: mmu: Allocate and free TDP MMU roots > kvm: mmu: Add functions to handle changed TDP SPTEs > kvm: mmu: Make address space ID a property of memslots > kvm: mmu: Support zapping SPTEs in the TDP MMU > kvm: mmu: Separate making non-leaf sptes from link_shadow_page > kvm: mmu: Remove disallowed_hugepage_adjust shadow_walk_iterator arg > kvm: mmu: Add TDP MMU PF handler > kvm: mmu: Factor out allocating a new tdp_mmu_page > kvm: mmu: Allocate struct kvm_mmu_pages for all pages in TDP MMU > kvm: mmu: Support invalidate range MMU notifier for TDP MMU > kvm: mmu: Add access tracking for tdp_mmu > kvm: mmu: Support changed pte notifier in tdp MMU > kvm: mmu: Add dirty logging handler for changed sptes > kvm: mmu: Support dirty logging for the TDP MMU > kvm: mmu: Support disabling dirty logging for the tdp MMU > kvm: mmu: Support write protection for nesting in tdp MMU > kvm: mmu: NX largepage recovery for TDP MMU > kvm: mmu: Support MMIO in the TDP MMU > kvm: mmu: Don't clear write flooding count for direct roots > > arch/x86/include/asm/kvm_host.h | 17 + > arch/x86/kvm/Makefile | 3 +- > arch/x86/kvm/mmu/mmu.c | 437 ++++++---- > arch/x86/kvm/mmu/mmu_internal.h | 98 +++ > arch/x86/kvm/mmu/paging_tmpl.h | 3 +- > arch/x86/kvm/mmu/tdp_iter.c | 198 +++++ > arch/x86/kvm/mmu/tdp_iter.h | 55 ++ > arch/x86/kvm/mmu/tdp_mmu.c | 1315 +++++++++++++++++++++++++++++++ > arch/x86/kvm/mmu/tdp_mmu.h | 52 ++ > include/linux/kvm_host.h | 2 + > virt/kvm/kvm_main.c | 7 +- > 11 files changed, 2022 insertions(+), 165 deletions(-) > create mode 100644 arch/x86/kvm/mmu/tdp_iter.c > create mode 100644 arch/x86/kvm/mmu/tdp_iter.h > create mode 100644 arch/x86/kvm/mmu/tdp_mmu.c > create mode 100644 arch/x86/kvm/mmu/tdp_mmu.h > Ok, I've not finished reading the code but I have already an idea of what it's like. I really think we should fast track this as the basis for more 5.11 work. I'll finish reviewing it and, if you don't mind, I might make some of the changes myself so I have the occasion to play and get accustomed to the code; speak up if you disagree with them though! Another thing I'd like to add is a few tracepoints. Paolo
