Abstract

Memory persistency models provide the foundational rules for software engineers to develop applications that take advantage of non-volatile memory (NVM), dictating which (and when) writes to NVM are deemed persistent. Though formalised for Intel-x86 and Arm architectures, these models remain empirically unvalidated on actual machines. Conventional validation methods for memory consistency models fall short as test programs cannot differentiate between volatile cache reads and those from NVM. To address this, we employed a commercial device designed to intercept and log data on a system’s memory bus in their order of arrival. We used this device to conduct a campaign using litmus tests—small programs designed to assess specific memory persistency behaviours—aimed at empirically validating Intel-x86 and Arm machine persistency guarantees.

We noted out-of-order memory writes and ensured they were not merely artifacts of our test setup. Analysis revealed Intel-x86’s architecture cannot be validated via memory bus interception due to legitimate early subsystem reordering. Intel engineers confirmed the absence of dependable validation methods for the persistency claims of their architectures. Meanwhile, an expert-recommended Arm machine did not align with the formal persistency model due to a specification loophole, and further investigation suggests that no market-available Arm machine fully supports NVM.

Our finding for Intel highlights a major concern for software developers wishing to take advantage of NVM: currently there is, to our knowledge, no viable way to confirm the persistency guarantees claimed by Intel. Our results for Arm suggest that our interceptionbased approach is viable for reliably detecting reorderings in the memory subsystem, which will be valuable for empirical validation once NVM-supporting machines become available.