In addition to Linux 5.17 bringing support for the low-cost StarFive RISC-V platform and other RISC-V updates, Friday also brought more changes to the licensing fee-free processor ISA. Among these latest RISC-V changes in Linux 5.17, the most notable is the provision of sv48 support, RISC-V sv48 refers to allowing 48-bit virtual address space support.
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With the fourth level paging table, the RISC-V 64-bit kernel can now address up to 128TB of virtual address space, corresponding to allowing 64TB of physical memory. Of course, we haven’t seen any high-end RISC-V server platforms yet capable of supporting anything close to the current limit – we don’t even see any high-capacity RAM RISC-V servers exist yet, but it’s a good idea for the future of RISC-V architecture development is obviously a good thing.
sv48 is described in the details in the RISC-V Instruction Set Manual Volume II: Privileged Architecture m v1.10:
Linux 5.17 can automatically detect support for sv48 at runtime and fall back to level 3 paging table support on non-sv48 hardware, Linux started writing patches for sv48 support going back at least 2020, before it was considered ready for mainline Previously, it had gone through multiple rounds of code reviews. Linux 5.17’s sv48 support and other final RISC-V additions will be part of this merge into the Linux kernel mainline.