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FOSDEM-RISCV-SBI-Atish.pdf
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2023-10-20
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FOSDEM_RISCV_SBI_Atish riscv sbi
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The future of RISC-V Supervisor
Binary Interface (SBI)
Atish Patra
Principal R&D Engineer
2/1/19©2018 Western Digital Corporation or its affiliates. All rights reserved.
• RISC-V Supervisor Binary Interface(SBI)
• Current status
• Limitations
• Extension proposal
• OpenSBI project
• Boot Flow
• Demo
Outline
2/1/19 2©2018 Western Digital Corporation or its affiliates. All rights reserved.
• Provides an interface to access machine mode only registers
• Clear separation between Supervisor & Supervisor Execution Environment (SEE)
• Helps to run single OS image across different SEE
• Currently implemented by the Berkeley Boot Loader (BBL)
• Calling convention
– S mode traps into M mode using ecall instructions
– Arguments are passed via registers a0-a2
– SBI call type is specified via register a7
– Unsupported SBI returns -38 (ENOSYS in Linux)
– a0 is clobbered register
• Documentation available at
– https://github.com/riscv/riscv-sbi-doc
RISC-V Supervisor Binary Interface (SBI)
2/1/19 3©2018 Western Digital Corporation or its affiliates. All rights reserved.
SEE
SBI
OS (S Mode)
ABI ABI
App1
App2
Current Interface
2/1/19 4©2018 Western Digital Corporation or its affiliates. All rights reserved.
Type
Function
Function
ID
Timer
sbi_set_timer
0
Console
sbi_console_putchar
sbi_console_getchar
1
2
IPI
sbi_clear_ipi
sbi_send_ipi
3
4
Memory
Model
sbi_remote_fence_i
sbi_remote_sfence_vma
sbi_remote_sfence_vma_as
id
5
6
7
Shutdown
sbi_shutdown
8
• Fixed, Not extendable
• No way to modify existing function signatures
• Changes cannot easily maintain backward compatibility
• No clean way to add new SBI function calls
– Power management
– Hart hotplug
– Vendor specific extensions
Limitations
2/1/19 5©2018 Western Digital Corporation or its affiliates. All rights reserved.
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