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Copyright (c) 2023 OpenHW Group Copyright (c) 2023 Thales DIS design services SAS

SPDX-License-Identifier: Apache-2.0 WITH SHL-2.1

This chapter is applicable to all configurations.

PMA

An underlying system can have multiple physical memory attributes (PMA). CVA6 supports three main access properties:

  • Non-idempotent regions (I/O regions): By the vary nature of a pipelined CPU architecture, the CPU will speculatively fetch (because of branch-prediction) and speculatively load (because of speculative execution). This can be problematic if reads to such a region are non-idempotent (i.e., they destroy state). As such, regions marked as non-idempotent will not be queried speculatively. Typical registers of that kind would be the UART or an interrupt claim register.
  • Executable regions (Main memory): Regions marked as executable will be considered as code regions. This will allow the core to fetch instructions from that region.
  • Cacheable regions (Main memory): Regions marked as cacheable will be considered as data and instruction regions. This will allow the core to fetch, load, and store data from that region.

The regions are instantiation-time parameters, they can not be changed during runtime. CVA6 uses the following fields in the cva6_cfg_t configuration structure to describe the PMA regions statically:

  • CVA6Cfg.NrNonIdempotentRules: Number of active non-idempotent regions.
    • CVA6Cfg.NonIdempotentAddrBase: Base address of the non-idempotent region.
    • CVA6Cfg.NonIdempotentLength: Length of the non-idempotent region.
  • CVA6Cfg.NrExecuteRegionRules: Number of active executable regions.
    • CVA6Cfg.ExecuteRegionAddrBase: Base address of the executable region.
    • CVA6Cfg.ExecuteRegionLength: Length of the executable region.
  • CVA6Cfg.NrCachedRegionRules: Number of active cacheable regions.
    • CVA6Cfg.CachedRegionAddrBase: Base address of the cacheable region.
    • CVA6Cfg.CachedRegionLength: Length of the cacheable region.

Unsupported PMAs

Currently the following RISC-V defined PMAs are not supported:

  • Coherence: Stand-alone CVA6 does not support any coherence protocol. All memory accesses are considered as non-coherent. For coherent systems, such as OpenPiton, it is up to the system to define coherence attributes exactly. Most likely this will mean any cacheable region is also coherent.
  • Atomicity: Atomicity is delegated to the underlying system. It is up the target to decide whether atomicity is supported or not.
  • Reservability: Reservability is delegated to the underlying system. It is up the target to decide whether atomicity is supported or not.
  • Vacant Regions: It is up to the underlying system to decide whether a region is vacant or not. CVA6 will not check for vacant regions. An AXI Decode Error or Slave Error is expected on vacant regions.