ARM-V8 PCIe Peer-to-Peer DMA Performance Drop Due to IOMMU_MMIO Attribute The core issue revolves around a significant performance degradation observed during PCIe peer-to-peer transactions between two GPU cards on an ARM-V8 server when the IOMMU is enabled. The throughput drops from an expected 28GB/s to a mere 4GB/s. This degradation is traced back to the…
ARMv9 RME Cache Coherency Problems During GPC-Protected Memory Access The ARMv9 architecture introduces Realm Management Extensions (RME), which include Granule Protection Checks (GPC) to enforce memory access permissions at a granular level. The GPC mechanism is designed to ensure that memory accesses are validated against the Granule Protection Table (GPT) before proceeding. However, a critical…
ARMv7-M Exception Handling and Stack Switching During Late Arriving Interrupts The ARMv7-M architecture, which includes popular cores like the Cortex-M3, Cortex-M4, and Cortex-M7, employs a sophisticated exception handling mechanism designed to ensure deterministic and efficient interrupt servicing. One of the key features of this architecture is the dual-stack mechanism, which utilizes the Main Stack Pointer…
GICv2 and GICv3 Priority Drop and Deactivation Race Condition Overview In hypervisor environments utilizing ARM’s Generic Interrupt Controller (GIC) versions 2 (GICv2) and 3 (GICv3), a race condition can occur between the priority drop and deactivation of physical interrupts when routing interrupts to virtual machines (VMs). This issue arises specifically when the hypervisor is configured…
ARM SVE Support in Apple M1 Pro: Architectural Overview and Limitations The Apple M1 Pro CPU, part of Apple’s custom silicon lineup, is based on the ARM architecture but diverges significantly from standard ARM implementations. While the M1 Pro supports ARM’s Advanced SIMD (Neon) technology, it does not incorporate ARM’s Scalable Vector Extension (SVE). This…
ARM Cortex-M JTAG Connectivity Failure During Debugging The error message "No Cortex-M device found in JTAG device chain" is a common issue encountered when attempting to debug or flash firmware onto an ARM Cortex-M microcontroller using a JTAG interface. This error indicates that the debug probe, such as J-Link, is unable to establish a connection…
ARM Cortex-M4 Hard Fault Due to Invalid EXC_RETURN and Stack Frame Manipulation The core issue revolves around a Hard Fault occurring on an ARM Cortex-M4 microcontroller when attempting to transition from Handler mode to Thread mode during nested interrupt handling. The fault is triggered by an invalid Program Counter (PC) load caused by an incorrect…
ARM PL390 GIC Priority Interpretation and Binary Point Register Configuration The ARM PL390 Generic Interrupt Controller (GIC) is a critical component in managing interrupts for ARM-based systems. One of the most nuanced aspects of the PL390 GIC is its priority handling mechanism, which is controlled by the Binary Point Register (ICCBPR). The ICCBPR splits the…
ARM Cortex-A8 L2 Cache Disabling Failure in Supervisor Mode The core issue revolves around the inability to disable the L2 cache on an ARM Cortex-A8 processor running in a bare-metal U-Boot environment on a BeagleBone Black. The user attempts to modify the Control Register (CP15, C1) to disable the L2 cache by clearing the C…
ARM GICv3 LPI Passthrough Behavior and State Machine The ARM Generic Interrupt Controller (GIC) version 3 introduces Locality-specific Peripheral Interrupts (LPIs), which are message-based interrupts designed for high-performance and scalable systems. Unlike traditional wired interrupts such as Peripheral Private Interrupts (PPIs) and Shared Peripheral Interrupts (SPIs), LPIs operate with a reduced state machine, which introduces…