| // Copyright 2021 The Fuchsia Authors |
| // |
| // Use of this source code is governed by a MIT-style |
| // license that can be found in the LICENSE file or at |
| // https://opensource.org/licenses/MIT |
| |
| #include <lib/arch/asm.h> |
| |
| .text |
| |
| // void LoadCodeSegmentSelector(uint16_t code_segment); |
| // |
| // Load the %cs segment selector with `code_segment`. |
| // |
| // This is hard to implement with inline assembly because the "lretq" |
| // instruction (used to perform a far jump, which in turn is required to |
| // update %cs) requires us to push/pop data to/from the stack. GCC/Clang don't |
| // give inline assembly freedom to modify the stack pointer (and attempting to |
| // do so can break compiler red zone optimisations, where the compiler performs |
| // reads/writes beyond the stack pointer). |
| // |
| .function LoadCodeSegmentSelector, global |
| |
| // Set %cs to `code_segment`. |
| // |
| // We do this by doing a far return operation (which pops the new %rip and |
| // new %cs off the stack) to the label "1". |
| leaq 1f(%rip), %rax |
| pushq %rdi // `code_segment` |
| .cfi_adjust_cfa_offset 8 |
| pushq %rax // new PC |
| .cfi_adjust_cfa_offset 8 |
| lretq |
| .cfi_adjust_cfa_offset -16 |
| 1: |
| retq |
| |
| .end_function |
