[RISC-V Architecture Training] @DEMO: PK (proxy kernel) and FESVR (front-end server)

riscv

PK and FESVRV

  • PK (proxy kernel) & FESVR (front-end server)
    • For debug and system bring up
    • PK is an abstraction of kernel that provides system services through FESVR running on host

pic

  • PK is running on target CPU, while FESVR is running on host computer

pic

PK and FESVR / code example

printf in modified version of spike

*# Original spike
> cd ~/riscv-training/lab/22-lab.system-call
> spike ../pk hello.elf
bbl loader
Hello world!

*# Modified spike (print out all system call)
> cd ~/riscv-training/lab/22-lab.system-call
> ~/riscv-git/riscv-tools/riscv-isa-sim/build/spike ../pk hello.elf
bbl loader
fesvr::sys_getmainvars (0x 8000d860 200 0 0 0 0 0)
fesvr::sys_openat (0x ffffffffffffff9c 8000d8b5 a 0 0 0 0)
fesvr::sys_pread (0x 3 8000d600 40 0 0 0 0)
...
fesvr::sys_pread (0x 3 80830000 1000 a000 0 0 0)
fesvr::sys_fstat (0x 1 80024df0 0 0 0 0 0)
fesvr::sys_pread (0x 3 8082c000 1000 6000 0 0 0)
fesvr::sys_write (0x 1 80834230 d 0 0 0 0)
Hello world!
fesvr::sys_exit (0x 0 0 0 0 0 0 0)

PK and FESVR / system call

tohost and fromhost

  • Memory location: shared knowledge between PK and FESVR
  • Both are 32-bit size, that can be read/write with single access
  • Follow producer-consumer model
    • tohost is written by PK, cleared by FESVR
    • fromhost is written by FESVR, cleared by PK

magicmem

  • tohost and fromhost are too small to communicate real data structure. They only store the address of magicmem
  • Syscall type, arguments and return values are stored in magicmem

Syscall entry point in PK

~/riscv-git/riscv-tools/riscv-pk/pk/syscall.c

Syscall handler in FESVR

~/riscv-git/riscv-tools/riscv-isa-sim/fesvr/syscall.cc

PK and FESVR / system call workflow

Please remember, this is for debugging hardware and system bring-up, sometimes co-processor running enviroment. And it’s good for understanding application/kernel interaction.

Target side (PK on RISC-V) Host side (FESVR on x86)
User-level code: ecall and trap into PK
Write syscall arguments into magicmem
Write address of magicmem into tohost Looping: read tohost until it’s non-0
Read magicmem
Looping: read tohost until it’s 0 Write 0 to tohost
Deal with syscall. Write return values into magicmem
Looping: read fromhost until it’s non-0 Write address of magicmem into fromhost
Read magicmem
Write 0 to fromhost Looping: read fromhost until it’s 0

PK and FESVR / verification exit

Another very useful scenario

  • To pass exit code in verification. It’s embedded inside the riscv-tests verification suite.
  • RVTEST_PASS and RVTEST_FAIL in ~/riscv-git/riscv-tools/riscv-tests/env/p/riscv_test.h
#define RVTEST_PASS                                                     \
        fence;                                                          \
        li TESTNUM, 1;                                                  \
        ecall

#define TESTNUM gp
#define RVTEST_FAIL                                                     \
        fence;                                                          \
1:      beqz TESTNUM, 1b;                                               \
        sll TESTNUM, TESTNUM, 1;                                        \
        or TESTNUM, TESTNUM, 1;                                         \
        ecall

@LAB: new system call

Add new system call to synchronize system time between target and host

Read CSR of mtime to get system time on RISC-V CPU

Need to use assembly code to get mtime