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Add some documentation on syscalls and argument handling

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Toby Jaffey 2025-12-16 22:04:50 +00:00
parent 4d3b393205
commit 8c68d48f80
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doc/README.md
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@ -95,46 +95,6 @@ When the external RAM is written to by the VM, the dirty flag will be set. The f
bool uvm32_extramDirty(uvm32_state_t *vmst)
## syscall ABI
All communication between bytecode and the vm host is performed via syscalls.
To make a syscall, register `a7` is set with the syscall number (a `UVM32_SYSCALL_x`) and `a0`, `a1` are set with the syscall parameters. The response is returned in `a2`.
[target.h](common/uvm32_target.h#L12)
```c
static uint32_t syscall(uint32_t id, uint32_t param1, uint32_t param2) {
register uint32_t a0 asm("a0") = (uint32_t)(param1);
register uint32_t a1 asm("a1") = (uint32_t)(param2);
register uint32_t a2 asm("a2");
register uint32_t a7 asm("a7") = (uint32_t)(id);
asm volatile (
"ecall"
: "=r"(a2) // output
: "r"(a7), "r"(a0), "r"(a1) // input
: "memory"
);
return a2;
}
```
The [RISC-V SBI](https://github.com/riscv-non-isa/riscv-sbi-doc/blob/master/riscv-sbi.adoc) is not followed, a simpler approach is taken.
## syscalls
There are two inbuilt syscalls used by uvm32, `halt()` and `yield()`.
`halt()` tells the host that the program has ended normally. `yield()` tells the host that the program requires more instructions to be executed. Halt is handled internally and transitions the VM to `UVM32_STATUS_ENDED`, `yield()` is handled in the VM host like other syscalls.
Syscalls are handled in the host by reading the syscall identifier, then using the provided functions to get arguments and set a return response. Direct access to the VM's memory space is not allowed, to avoid memory corruption issues.
The following functions are used to access syscall parameters safely:
uint32_t uvm32_getval(uvm32_state_t *vmst, uvm32_evt_t *evt, uvm32_arg_t);
const char *uvm32_getcstr(uvm32_state_t *vmst, uvm32_evt_t *evt, uvm32_arg_t);
void uvm32_setval(uvm32_state_t *vmst, uvm32_evt_t *evt, uvm32_arg_t, uint32_t val);
uvm32_evt_syscall_buf_t uvm32_getbuf(uvm32_state_t *vmst, uvm32_evt_t *evt, uvm32_arg_t argPtr, uvm32_arg_t argLen);
## Event driven operation