Hi, On 2020/6/16 17:35, Keqian Zhu wrote: > This patch series add support for stage2 hardware DBM, and it is only > used for dirty log for now. > > It works well under some migration test cases, including VM with 4K > pages or 2M THP. I checked the SHA256 hash digest of all memory and > they keep same for source VM and destination VM, which means no dirty > pages is missed under hardware DBM. > > Some key points: > > 1. Only support hardware updates of dirty status for PTEs. PMDs and PUDs > are not involved for now. > > 2. About *performance*: In RFC patch, I have mentioned that for every 64GB > memory, KVM consumes about 40ms to scan all PTEs to collect dirty log. > > Initially, I plan to solve this problem using parallel CPUs. However > I faced two problems. > > The first is bottleneck of memory bandwith. Single thread will occupy > bandwidth about 500GB/s, we can support about 4 parallel threads at > most, so the ideal speedup ratio is low. Aha, I make it wrong here. I test it again, and find that speedup ratio can be about 23x when I use 32 CPUs to scan PTs (takes about 5ms when scanning PTs of 200GB RAM). > > The second is huge impact on other CPUs. To scan PTs quickly, I use > smp_call_function_many, which is based on IPI, to dispatch workload > on other CPUs. Though it can complete work in time, the interrupt is > disabled during scaning PTs, which has huge impact on other CPUs. I think we can divide scanning workload into smaller ones, which can disable and enable interrupts periodly. > > Now, I make hardware dirty log can be dynamic enabled and disabled. > Userspace can enable it before VM migration and disable it when > remaining dirty pages is little. Thus VM downtime is not affected. BTW, we can reserve this interface for userspace if CPU computing resources are not enough. Thanks, Keqian > > > 3. About correctness: Only add DBM bit when PTE is already writable, so > we still have readonly PTE and some mechanisms which rely on readonly > PTs are not broken. > > 4. About PTs modification races: There are two kinds of PTs modification. > > The first is adding or clearing specific bit, such as AF or RW. All > these operations have been converted to be atomic, avoid covering > dirty status set by hardware. > > The second is replacement, such as PTEs unmapping or changement. All > these operations will invoke kvm_set_pte finally. kvm_set_pte have > been converted to be atomic and we save the dirty status to underlying > bitmap if dirty status is coverred. > > > Keqian Zhu (12): > KVM: arm64: Add some basic functions to support hw DBM > KVM: arm64: Modify stage2 young mechanism to support hw DBM > KVM: arm64: Report hardware dirty status of stage2 PTE if coverred > KVM: arm64: Support clear DBM bit for PTEs > KVM: arm64: Add KVM_CAP_ARM_HW_DIRTY_LOG capability > KVM: arm64: Set DBM bit of PTEs during write protecting > KVM: arm64: Scan PTEs to sync dirty log > KVM: Omit dirty log sync in log clear if initially all set > KVM: arm64: Steply write protect page table by mask bit > KVM: arm64: Save stage2 PTE dirty status if it is coverred > KVM: arm64: Support disable hw dirty log after enable > KVM: arm64: Enable stage2 hardware DBM > > arch/arm64/include/asm/kvm_host.h | 11 + > arch/arm64/include/asm/kvm_mmu.h | 56 +++- > arch/arm64/include/asm/sysreg.h | 2 + > arch/arm64/kvm/arm.c | 22 +- > arch/arm64/kvm/mmu.c | 411 ++++++++++++++++++++++++++++-- > arch/arm64/kvm/reset.c | 14 +- > include/uapi/linux/kvm.h | 1 + > tools/include/uapi/linux/kvm.h | 1 + > virt/kvm/kvm_main.c | 7 +- > 9 files changed, 499 insertions(+), 26 deletions(-) >
