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Subspace Snooping: Exploiting Temporal Sharing Stability for Snoop Reduction
Jeongseob Ahn,Daehoon Kim,Jaehong Kim,Jaehyuk Huh IEEE 2012 IEEE Transactions on Computers Vol.61 No.11
<P>Although snoop-based coherence protocols provide fast cache-to-cache transfers with a simple and robust coherence mechanism, scaling the protocols has been difficult due to the overheads of broadcast snooping. In this paper, we propose a coherence filtering technique called subspace snooping, which stores the potential sharers of each memory page in the page table entry. By using the sharer information in the page table entry, coherence transactions for a page generate snoop requests only to the subset of nodes in the system. However, the coherence subspace of a page may evolve, as the phases of applications may change or the operating system may migrate threads to different nodes. To adjust subspaces dynamically, subspace snooping supports two different shrinking mechanisms, which remove obsolete nodes from subspaces. Among the two shrinking mechanisms, subspace snooping with safe shrinking can be integrated to any type of coherence protocols and network topologies, as it guarantees that a subspace always contains the precise sharers of a page. Speculative shrinking breaks the subspace superset property, but achieves better snoop reductions than safe shrinking. We evaluate subspace snooping with Token Coherence on unordered mesh networks. Subspace snooping reduces 58 percent of snoops on average for a set of parallel scientific and server workloads, and 87 percent for our multiprogrammed workloads.</P>
Fast Two-Level Address Translation for Virtualized Systems
Jeongseob Ahn,Seongwook Jin,Jaehyuk Huh IEEE 2015 IEEE Transactions on Computers Vol. No.
<P>Recently, there have been several improvements in architectural supports for two-level address translation for virtualized systems. However, those improvements including HW-based two-dimensional (2D) page walkers have extended the traditional multi-level page tables, without considering the memory management characteristics of virtual machines. This paper exploits the unique behaviors of the hypervisor, and proposes three new nested address translation schemes for virtualized systems. The first scheme called nested segmentation is designed for static memory allocation, and uses HW segmentation to map the VM memory directly to large contiguous memory regions. The second scheme proposes to use a flat nested page table for each VM, reducing memory accesses by the current 2D page walkers. The third scheme uses speculative inverted shadow paging, backed by non-speculative flat nested page tables. The speculative mechanism provides direct translation with a single memory reference for common cases without page table synchronization overheads. We evaluate the proposed schemes with the Xen hypervisor running on a full system simulator. Nested segmentation can reduce the overheads of two-level translation significantly for a certain cloud computing model. The nested segmentation, flat page tables, and speculative shadowing improve a state-of-the-art 2D page walker by 10, 7, and 14 percent respectively.</P>
H-SVM: Hardware-Assisted Secure Virtual Machines under a Vulnerable Hypervisor
Seongwook Jin,Jeongseob Ahn,Jinho Seol,Sanghoon Cha,Jaehyuk Huh,Seungryoul Maeng IEEE 2015 IEEE Transactions on Computers Vol.64 No.10
<P>With increasing demands on cloud computing, protecting guest virtual machines (VMs) from malicious attackers has become critical to provide secure services. The current cloud security model with software-based virtualization relies on the invulnerability of the software hypervisor and its trustworthy administrator with the root permission. However, compromising the hypervisor with remote attacks or root permission grants the attackers with a full access capability to the memory and context of a guest VM. This paper proposes a HW-based approach to protect guest VMs even under an untrusted hypervisor. With the proposed mechanism, memory isolation is provided by the secure hardware, which is much less vulnerable than the software hypervisor. The proposed mechanism extends the current hardware support for memory virtualization based on nested paging with a small extra hardware cost. The hypervisor can still flexibly allocate physical memory pages to virtual machines for efficient resource management. In addition to the system design for secure virtualization, this paper presents a prototype implementation using system management mode. Although the current system management mode is not intended for security functions and thus limits the performance and complete protection, the prototype implementation proves the feasibility of the proposed design.</P>
ODAR: Aerial Manipulation Platform Enabling Omnidirectional Wrench Generation
Park, Sangyul,Lee, Jeongseob,Ahn, Joonmo,Kim, Myungsin,Her, Jongbeom,Yang, Gi-Hun,Lee, Dongjun IEEE 2018 IEEE/ASME transactions on mechatronics Vol.23 No.4
<P>We propose a novel aerial manipulation platform, an omnidirectional aerial robot, that is capable of omnidirectional wrench generation with opportunistically distributed/aligned Sectional rotors. To circumvent the tight thrust margin and weight budget of currently available rotor and battery technologies, we propose a novel design optimization framework, which maximizes the minimum-guaranteed control force/torque for any attitude while incorporating such important and useful aspects as interrotor aerointerference, anisotropic task requirement, gravity compensation, etc. We also provide a closed-form solution of infinity-norm optimal control allocation to avoid rotor saturation with the tight thrust margin. Further, we elaborate the notion of electronic speed controller induced singularity and devise a novel selective mapping algorithm to substantially subdue its destabilizing effect. Experiments are performed to validate the theory, which demonstrate such capabilities not possible with typical aerial manipulation systems, namely, separate translation and attitude control on SE(3), hybrid pose/wrench control with downward force of 60 N much larger than its own weight (2.6 kg), and peg-in-hole teleoperation with a radial tolerance of 0.5 mm.</P>