| use core::hint; |
| |
| /// Tock programs' entry point. Called by the kernel at program start. Sets up |
| /// the stack then calls rust_start() for the remainder of setup. |
| #[doc(hidden)] |
| #[naked] |
| #[no_mangle] |
| #[link_section = ".start"] |
| // The args for this function are: |
| // app_start: usize, |
| // mem_start: usize, |
| // memory_len: usize, |
| // app_heap_break: usize, |
| // Due to Rust issue: https://github.com/rust-lang/rust/issues/42779 we can't have |
| // args to the function |
| pub unsafe extern "C" fn _start() -> ! { |
| asm!( |
| // Compute the stack top. |
| // |
| // struct hdr* myhdr = (struct hdr*) app_start; |
| // uint32_t stacktop = (((uint32_t) mem_start + myhdr->stack_size + 7) & 0xfffffff8); |
| "lw t0, 36(a0) // t0 = myhdr->stack_size |
| addi t0, t0, 7 // t0 = myhdr->stack_size + 7 |
| add t0, t0, a1 // t0 = mem_start + myhdr->stack_size + 7 |
| li t1, 7 // t1 = 7 |
| not t1, t1 // t1 = ~0x7 |
| and t0, t0, t1 // t0 = (mem_start + myhdr->stack_size + 7) & ~0x7 |
| // |
| // Compute the app data size and where initial app brk should go. |
| // This includes the GOT, data, and BSS sections. However, we can't be sure |
| // the linker puts them back-to-back, but we do assume that BSS is last |
| // (i.e. myhdr->got_start < myhdr->bss_start && myhdr->data_start < |
| // myhdr->bss_start). With all of that true, then the size is equivalent |
| // to the end of the BSS section. |
| // |
| // uint32_t appdata_size = myhdr->bss_start + myhdr->bss_size; |
| lw t1, 24(a0) // t1 = myhdr->bss_start |
| lw t2, 28(a0) // t2 = myhdr->bss_size |
| lw t3, 4(a0) // t3 = myhdr->got_start |
| add t1, t1, t2 // t1 = bss_start + bss_size |
| // |
| // Move arguments we need to keep over to callee-saved locations. |
| mv s0, a0 // s0 = void* app_start |
| mv s1, t0 // s1 = stack_top |
| mv s2, a3 // s2 = app_heap_break |
| // |
| // Now we may want to move the stack pointer. If the kernel set the |
| // `app_heap_break` larger than we need (and we are going to call `brk()` |
| // to reduce it) then our stack pointer will fit and we can move it now. |
| // Otherwise after the first syscall (the memop to set the brk), the return |
| // will use a stack that is outside of the process accessible memory. |
| // |
| bgt t1, a3, skip_set_sp // Compare `app_heap_break` with new brk. |
| // If our current `app_heap_break` is larger |
| // then we need to move the stack pointer |
| // before we call the `brk` syscall. |
| mv sp, t0 // Update the stack pointer |
| |
| skip_set_sp: // Back to regularly scheduled programming. |
| |
| // Call `brk` to set to requested memory |
| |
| // memop(0, stacktop + appdata_size); |
| li a0, 4 // a0 = 4 // memop syscall |
| li a1, 0 // a1 = 0 |
| mv a2, t1 // a2 = appdata_size |
| ecall // memop |
| // |
| // Debug support, tell the kernel the stack location |
| // |
| // memop(10, stacktop); |
| li a0, 4 // a0 = 4 // memop syscall |
| li a1, 10 // a1 = 10 |
| mv a2, s1 // a2 = stacktop |
| ecall // memop |
| // |
| // Debug support, tell the kernel the heap location |
| // |
| // memop(11, stacktop + appdata_size); |
| li a0, 4 // a0 = 4 // memop syscall |
| li a1, 11 // a1 = 11 |
| mv a2, t1 // a2 = appdata_size |
| ecall // memop |
| // |
| // Setup initial stack pointer for normal execution |
| // Call into the rest of startup. This should never return. |
| mv sp, s1 // sp = stacktop |
| mv a0, s0 // first arg is app_start |
| mv s0, sp // Set the frame pointer to sp. |
| mv a1, s1 // second arg is stacktop |
| mv a2, t1 // third arg is app_heap_break that we told the kernel |
| jal rust_start" |
| : // No output operands |
| : |
| : "memory", "x10", "x11", "x12", "x13", "x14", "x15", "x16", "x17", |
| "x5", "x6", "x7", "x28", "x29", "x30", "x31", "x1" // Clobbers |
| : "volatile" // Options |
| ); |
| hint::unreachable_unchecked(); |
| } |
| |
| /// Ensure an abort symbol exists. |
| #[cfg(target_arch = "riscv32")] |
| #[link_section = ".start"] |
| #[export_name = "abort"] |
| pub extern "C" fn abort() { |
| unsafe { |
| asm! (" |
| // Simply go back to the start as if we had just booted. |
| j _start |
| " |
| : |
| : |
| : |
| : "volatile"); |
| } |
| } |