FOSSIL: Operating System support to leverage byte-granular Non-Volatile Memory Technology

新的存储技术，如支持字节粒度访问的非易失性存储器（NVM），被认为是破坏性的。特别是，因为它们能够以完全相同的方式访问易失性和非易失性存储器。然而，NVM对操作系统提出了挑战，因为它混合了当前非易失性存储设备（例如ssd）和当前易失性存储设备（例如DRAM）的属性：ssd像NVM一样是持久的，但是通过操作系统提供的文件系统以块为单位间接访问。像NVM一样，DRAM被映射到应用程序的地址空间中，但是没有操作系统提供的抽象。像处理ssd那样处理NVM会削弱它的一些优势，而像处理DRAM那样处理NVM会使操作系统无法有效地提供具有某些保证的抽象（例如，断电后的数据一致性）。幸运的是，另一个技术趋势是以各种形式（苹果M1、安腾）或硬件能力（CHERI、ARM Morello）引入“快速通话”。这些快速调用避免了切换到操作系统内核的昂贵代价，但仍然允许操作系统对资源访问强制执行某些属性。出于这个原因，快速调用是解决操作系统面临的NVM挑战的一个有希望的候选：操作系统可以将NVM映射到应用程序的地址空间，并让应用程序使用快速调用来有效地执行特定的结构。我们计划构建两个用例：1)受保护的共享文件系统，以及2)在间歇性电源供应的情况下支持进程的快速恢复。为了使我们的研究保持简单的起点，我们使用了由项目PI团队开发的微内核技术，该技术现在已经足够成熟，可以部署在重要的工业应用（例如，汽车）和安全关键应用领域。在该项目中，我们将扩展一个类似l4的功能系统，使其能够在停电导致的易失性信息丢失中存活下来。为此，我们将探讨是否可以使用快速调用来检查细粒度的功能权限，而无需昂贵的内核调用。此外，我们将研究一种持久性设计，其中持久性和易失性表示必须在停电的情况下保持某些不变性。

Novel storage technologies like non-volatile memory (NVM) with support for byte-granular accesses are considered disruptive. In particular, because they enable the access of volatile and non-volatile memory in the exact same manner. However, NVM poses a challenge for OSes, because it mixes properties of current non-volatile storage devices (e.g., SSDs) and current volatile storage devices (e.g., DRAM): SSDs are persistent like NVM, but are accessed block-wise and indirectly via an OS-provided file system. DRAM is mapped into the applications' address space, like NVM, but is used without an OS-provided abstraction. Handling NVM like SSDs would mute some of its advantages, whereas the handling of NVM like DRAM would remove the ability for the OS to provide abstractions with certain guarantees efficiently (e.g., data consistency also after a power outage).Fortunately, another technology trend is the introduction of "fast calls" in various forms (Apple M1, Itanium) or hardware capabilities (CHERI, ARM Morello). These fast calls avoid the expensive switch to the OS kernel, but still allow the OS to enforce certain properties for resource accesses. For that reason, fast calls are a promising candidate to address the challenge that OSes face with NVM: OSes can map NVM into the application's address space and let applications use fast calls to efficiently enforce a specific structure.We plan to build two use cases: 1) a protected shared file system, and 2) support for rapid recovery of processes in case of intermittent power supplies. To keep the starting point for our research simple, we use microkernel technology developed by the team of the project's PI, which by now has become mature enough to be deployed in important industrial applications (e.g., automotive) and security-critical application areas. In the project, we will extend an L4-like capability system in such a way that it survives loss of volatile information caused by power outage. To this end, we explore if fast calls can be used to check fine-granular capability permissions without expensive kernel invocations. In addition, we will study a persistency design where persistent and volatile representations have to maintain certain invariants in case of power outages.