Open source firmware conference 2019

1. © 2019 Western Digital Corporation or its affiliates. All rights reserved. 8/30/19
The Role of Open Source
Firmware in RISC-V
Atish Patra <Atish.Patra@wdc.com>
Alistair Francis <Alistair.Francis@wdc.com>
Anup Patel <Anup.Patel@wdc.com>
OSFC – 3rd
of September 2019

2. 8/30/19© 2019 Western Digital Corporation or its affiliates. All rights reserved. 2
Outline
•Introduction to RISC-V
•Industry standard Open source bootloaders
•Current RISC-V boot flow
•OpenSBI Project
•Upstream Status
•Future work
•Demo

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Acknowledgements
•U-Boot
– Lukas Auler
– Bin Meng
– Anup Patel
•Coreboot
– Ron Minnich
– Patrick Rudolph
– Shawn Chang
•EDK2
– Abner Chang
• And others

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What is RISC-V?
•Pronounced as risk-five
•An open RISC Instruction Set Architecture (ISA)
•The base ISA is incredibly small
– The ISA can be extended
• Integer (I) and Embedded (E) are part of the base ISA
• Multiply/Divide (M), Atomic (A), Single Floating Point (F), Double Floating Point (D), Quad Floating Point (Q) and
Compressed Instructions (C) are frozen already
• Virtualisation (H), Vector (V), Bit Manipulation (B) are examples of draft extensions
• The General Extension (G) is shorthand for M, A, F and D
– Aimed at everything from small embedded systems to high end HPC
•Multiple CPU implementations already available
•SiFive, BOOM, Rocket64, Andes, QEMU, etc
•Currently heavily backed by SiFive, Microsemi, NVIDIA, Western Digital and others

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Why RISC-V?
•An open source ISA allows anyone to use and modify the ISA without any licensing cost
– This is different to commercial companies which ship black boxes with strict NDAs
•Customization for specific applications
•Security audits to verify security in critical applications
•Lower cost users (such as small companies or hobbyists) to use the ISA
•Community driven development approach allows input from anybody
– The RISC-V foundation still decides on the final specifications

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Industry standard boot loaders
ROM LOADER RUNTIME BOOTLOADER OS
Loads
Jumps
• Runs from On-Chip
ROM
• Uses On-Chip SRAM
• SOC power-up and
clock setup
• DDR initialization
• Loads RUNTIME and
BOOTLOADER
• Examples:
• BIOS/UEFI
• U-Boot SPL
• Coreboot ROM stage
• Runs from On-Chip SRAM or
DDR
• SOC security setup
• Runtime services as-per
specifications
• Examples:
• ATF
• BIOS/UEFI
• Runs from DDR
• Typically open-source
• Filesystem support
• Network booting
• Boot configuration
• Examples:
• U-Boot
• GRUB
• LinuxBoot
Proprietary
Open
Source

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RISC-V Upstream Boot Flow
Follows commonly used multiple boot stages model
• Follows a standard boot flow
• Uses U-Boot as the last stage boot loader
• U-Boot binary as the payload to OpenSBI
• FSBL is SiFive specific and will be replaced by Coreboot/U-Boot SPL
• OpenSBI is a RISC-V specific runtime service provider
Linux
(S-mode)
(OS)
ZSBL
(M-mode)
(ROM)
FSBL
(M-mode)
(LOADER)
OpenSBI
(M-mode)
(RUNTIME)
U-Boot
(S-mode)
(BOOTLOADER)
Loads
Jumps
Proprietary
Open
Source
RISC-V
specific

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What is SBI ?
•SBI is the RISC-V Supervisor Binary Interface
– System call style calling convention between Supervisor (S-mode OS) and Supervisor
Execution Environment (SEE)
•SEE can be:
– A M-mode RUNTIME firmware for OS/Hypervisor running in HS-mode
– A HS-mode Hypervisor for Guest OS running in VS-mode
•SBI calls help:
– Reduce duplicate platform code across OSes (Linux, FreeBSD, etc)
– Provide common drivers for an OS which can be shared by multiple platforms
– Provide an interface for direct access to hardware resources (M-mode only resources)
•Specifications being drafted by the Unix Platform Specification Working group
– Maintain and evolve the SBI specifications
– Currently, SBI v0.1 in-use and SBI v0.2 in draft stage
SEE (M-Mode)
SBI
OS (S-Mode)
ABI ABI
App1 App2

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What is OpenSBI ?
•OpenSBI is an open-source implementation of the RISC-V Supervisor Binary Interface
(SBI) specifications
– Licensed under the terms of the BSD-2 clause license
– Helps to avoid SBI implementation fragmentation
– Maintained by the community
•Aimed at providing RUNTIME services in M-mode
– Typically used in boot stage following ROM/LOADER
•Provides support for reference platforms
– Generic simple drivers included for M-mode to operate
▪ PLIC, CLINT, UART 8250
– Other platforms can reuse the common code and add needed drivers

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Key Features
•Layered structure to accommodate various use cases
– Generic SBI library with platform abstraction
• Typically used with external firmware and bootloader
– EDK2 (UEFI implementation), Secure boot working group
– Platform specific library
• Similar to core library but including platform specific drivers
– Platform specific reference firmware
• Three different types of RUNTIME firmware
•Wide range of hardware features supported
– RV32 and RV64
– Misaligned load/store handling
– Missing CSR emulation
– Protects firmware using PMP support
Platform Specific
Reference Firmware
Platform Specific
Library
SBI Library
OpenSBI Layers

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Reference Firmwares
•OpenSBI provides several types of reference firmware, all platform-specific
– FW_PAYLOAD
• Firmware with the next booting stage as a payload
• Default firmware being used in Linux capable RISC-V hardware
– FW_JUMP
• Firmware with static jump address to the next booting stage
• Default method for QEMU
– FW_DYNAMIC
• Firmware with dynamic information on the next booting stage
• U-Boot SPL is using FW_DYNAMIC
•SOC Vendors may choose:
– Use one of OpenSBI reference firmwares as their M-mode RUNTIME firmware
– Build M-mode RUNTIME firmware from scratch with OpenSBI as library
– Extend existing M-mode firmwares (U-Boot_M_mode/EDK2) with OpenSBI as library

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Platform support
•SiFive HiFive Unleashed
•Andes AE350
•Ariane FPGA SOC
•Kendryte K210
•QEMU virt machines (32-bit/64-bit)

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Upstream status
•OpenSBI
– Actively developed and maintained
– Version 0.4 released
– Default in Buildroot, Yocto/OpenEmbedded and
the QEMU “BIOS”
– Fedora/Debian provides images available with
OpenSBI binary
•U-Boot
– U-Boot-2019.07 release has HiFive Unleashed
S-mode support with SMP
– Boot via network supported
– MMC boot support coming in 2019.10
– EFI support for RISC-V available
•Grub
– RISC-V support available upstream
•Linux Kernel
– Upstream kernel boots in QEMU
– 5.3 kernel works with OpenSBI+U-Boot
on HiFive Unleashed
Rapid progress: traditional full boot support expected by year end

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Planned boot flow using U-Boot SPL
•OpenSBI firmware with dynamic information about the next booting
stage
•Coreboot support available on both hardware & QEMU
•U-Boot SPL Only available for QEMU for now
•HiFive Unleashed U-Boot SPL support not available yet
Passed via ‘a2’ register
Linux
(S-mode)
(OS)
ZSBL
(M-mode)
(ROM)
U-Boot SPL
Coreboot
(M-mode)
OpenSBI
(M-mode)
(RUNTIME)
U-Boot
(S-mode)
(BOOTLOADER)
Loads
Jumps
Proprietary
Open
Source
RISC-V
specific
struct
fw_dynamic_info

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Planned boot flow using EDK2
•Work in progress
•Open Source EDK2 (UEFI implementation) OpenSBI integration
– HPE leading this effort
– Available in EDK2 mailing list for U540 on Xilinx VC707 FPGA
– OpenSBI built with EDK2 build environment
•OpenSBI used as library in Pre-EFI Initialization (PEI) Phase
•Driver eXecution Environment (DXE) Phase in S-mode will use OpenSBI function
wrappers to make ecalls (Not done yet)

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Future boot support
•EFI stub support in Linux kernel for full UEFI support
– Enterprise class boot environment
•U-Boot SPL support for hardware
•Booting from SDCARD in coreboot possible ?
•EDK2 project upstreaming
•LinuxBoot ?
•Other bootloaders ?
Toward a stable and easy to use boot ecosystem

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Ongoing work
•SBI specifications
– SBI v0.2 specification
– Power management extension in SBI
•OpenSBI
– SBI v0.2 support
– Sequential cpu boot via CPU hotplug
– Support other M-mode boot loader such as
Coreboot/U-Boot SPL
– Hypervisor support
– More platforms support
• Need hardware !
Toward a stable and easy to use boot ecosystem

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OSTree
•OSTree can be thought of as git for operating system updates
•It is used to mange Linux system operating system updates
•Unlike git OSTree “checks out” files using hardlinks, so the rootFS is immutable to
prevent corruption
•OSTree started with Gnome Continuous (high performance continuous delivery/testing
system for GNOME)
•OSTree is used in meta-updater (an OE layer) and Fedora SilverBlue
•Flatpak uses libostree

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Demo 1: U-Boot TFTP boot and OSTree

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Demo 2: U-Boot MMC boot and OSTree

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Thank you!!
•Q&A ?

22. © 2019 Western Digital Corporation or its affiliates. All rights reserved. 8/30/19

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Constraints on using OpenSBI Library
•Same GCC target options (i.e. -march, -mabi, and -mcmodel) need to be used for the
external firmware and OpenSBI sources
•External firmware must create per-HART non-overlapping:
1. Program Stack
2. OpenSBI scratch space (i.e. struct sbi_scratch instance with extra space above)
•Two constraints in calling any OpenSBI functions from external firmware:
1. MSCRATCH CSR of calling HART must be set to its own OpenSBI scratch space
2. SP register (i.e. the stack pointer) of calling HART must be set to its own stack
•External firmware must also ensure that:
– Interrupts are disabled in the MSTATUS and MIE CSRs when calling sbi_init()
– sbi_init() is called for each HART that is powered-up at boot-time or in response to a CPU hotplug event
– sbi_trap_handler() is called for M-mode interrupts and M-mode traps

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Reference
•OpenSBI
– https://github.com/riscv/opensbi
•SBI
– https://github.com/riscv/riscv-sbi-doc
•EDK2
– https://edk2.groups.io/g/devel/message/46479?p=,,,20,0,0,0::Created,,riscv,20,2,0,33047245

25. 8/30/19© 2019 Western Digital Corporation or its affiliates. All rights reserved. 25
Planned boot flow using EDK2
•OpenSBI used as library
•Open Source EDK2 (UEFI implementation) OpenSBI integration
– HPE leading this effort
– Available in EDK2 staging area for U540 on Xilinx VC707 FPGA
– OpenSBI built with EDK2 build environment
Passed via ‘a2’ register
ZSBL
(M-mode)
(ROM)
Bootloader
(Grub/U-Bo
ot)
Loads
Jumps
Open
Source
RISC-V
specific
EDK2
FSBL
(M-mode)
(ROM)
Linux
(S-Mode)
OS
SEC
phase
PEI phase
(M-Mode)
DXE phase
(M-Mode)
BDS phase
(M-Mode)
EDK2 OpenSBI DXE Driver
(S-Mode)
OpenSBI library
(M-Mode)

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FW_PAYLOAD
•OpenSBI firmware with the next booting stage as a payload
– Any S-mode BOOTLOADER/OS image as the payload to OpenSBI FW_PAYLOAD
– Allows overriding device tree blob (i.e. DTB)
– Very similar to BBL hence fits nicely in existing boot-flow of SiFive Unleashed board
•Down-side:
– We have to re-create FW_PAYLOAD image whenever OpenSBI or the BOOTLOADER (U-Boot) changes
– No mechanism to pass parameters from previous booting stage (i.e. LOADER) to FW_PAYLOAD
FW_PAYLOAD
ROM
(M-mode)
(ZSBL)
LOADER
(M-mode)
(FSBL)
RUNTIME
(M-mode)
(OpenSBI)
OS
(S-mode)
(Linux)
Loads
Jumps
BOOTLOADER
(S-mode)
(U-Boot)

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FW_JUMP
•OpenSBI firmware with a fixed jump address to the next booting stage
– Next stage booting stage (i.e. BOOTLADER) and FW_JUMP are loaded by the previous booting stage
(i.e. LOADER)
– Very useful for QEMU because we can use pre-compiled FW_JUMP
•Down-side:
– Previous booting stage (i.e. LOADER) has to load next booting stage (i.e. BOOTLADER) at a fixed
location
– No mechanism to pass parameters from pervious booting stage (i.e. LOADER) to FW_JUMP
ROM
(M-mode)
(ZSBL)
LOADER
(M-mode)
(U-Boot_SPL/Coreboot/QEMU)
RUNTIME
(M-mode)
(FW_JUMP)
BOOTLOADER
(S-mode)
(U-Boot)
OS
(S-mode)
(Linux)
Loads
Jumps

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FW_DYNAMIC
struct fw_dynamic_info
unsigned long magic
unsigned long version
unsigned long next_addr
unsigned long next_mode
unsigned long options
•OpenSBI firmware with dynamic information about the next booting
stage
– The next stage booting stage (i.e. BOOTLADER) and FW_DYNAMIC are loaded by
the previous booting stage (i.e. LOADER)
– The previous booting stage (i.e. LOADER) passes the location of struct
fw_dynamic_info to FW_DYNAMIC via ‘a2’ register
•Down-side:
– Previous booting stage (i.e. LOADER) needs to be aware of struct fw_dynamic_info
ROM
(M-mode)
(ZSBL)
LOADER
(M-mode)
(U-Boot_SPL/Coreboot/QEMU)
RUNTIME
(M-mode)
(FW_DYNAMIC)
BOOTLOADER
(S-mode)
(U-Boot)
OS
(S-mode)
(Linux)
Loads
Jumps
struct
fw_dynamic_info
Passed via ‘a2’ register

29. 8/30/19© 2019 Western Digital Corporation or its affiliates. All rights reserved. 29
Planned boot flow using EDK2
•OpenSBI used as library
•Open Source EDK2 (UEFI implementation) OpenSBI integration
– HPE leading this effort
– Available in EDK2 staging area for U540 on Xilinx VC707 FPGA
– OpenSBI built with EDK2 build environment
Passed via ‘a2’ register
ZSBL
(M-mode)
(ROM)
Bootloader
(Grub/U-Bo
ot)
Loads
Jumps
Open
Source
RISC-V
specific
EDK2
FSBL
(M-mode)
(ROM)
Linux
(S-Mode)
OS
SEC
phase
PEI phase
(M-Mode)
DXE phase
(M-Mode)
BDS phase
(M-Mode)
EDK2 OpenSBI DXE Driver
(S-Mode)
OpenSBI library
(M-Mode)

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OpenSBI - Booting Instructions
QEMU
•Instructions available at opensbi/docs/platform/qemu_virt.md
•U-Boot image as a payload
– Building
– Running
HiFive Unleashed
• Instructions also available at
opensbi/docs/platform/sifive_fu540.md
• U-Boot binary as payload
make PLATFORM=qemu/virt
FW_PAYLOAD_PATH=<uboot_build_directory>/u-boot.bin
qemu-system-riscv64 -M virt -m 256M -display none -serial stdio
-kernel build/platform/qemu/virt/firmware/fw_payload.elf
make PLAT=sifive/hifive_u540
FW_PAYLOAD_PATH=~/workspace/u-boot-riscv/u-boot.bin