riscv64: Add unsafe comment for KVM_EXIT_RISCV_SBI

Signed-off-by: BillXiang <[email protected]>

Author: BillXiang

kvm-ioctls/src/ioctls/vcpu.rs

@@ -1694,6 +1694,8 @@ impl VcpuFd {
                 KVM_EXIT_HYPERV => Ok(VcpuExit::Hyperv),
                 #[cfg(target_arch = "riscv64")]
                 KVM_EXIT_RISCV_SBI => {
+                    // SAFETY: Safe because the exit_reason (which comes from the kernel) told us
+                    // which union field to use and the type of extension_id is 'enum sbi_ext_id'.
                     match unsafe {run.__bindgen_anon_1.riscv_sbi.extension_id} as u32 {
                         SBI_EXT_0_1_CONSOLE_PUTCHAR => {
                             let ch = unsafe { run.__bindgen_anon_1.riscv_sbi.args[0] };