safety: Add safety comment

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

Author: BillXiang

kvm-ioctls/src/ioctls/vcpu.rs

@@ -1698,10 +1698,14 @@ impl VcpuFd {
                     // 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 => {
+                            // SAFETY: Safe because the exit_reason (which comes from the kernel) told us
+                            // which union field to use
                             let ch = unsafe { run.__bindgen_anon_1.riscv_sbi.args[0] };
                             Ok(VcpuExit::SbiExt0_1ConsolePutchar(ch))
                         }
                         SBI_EXT_0_1_CONSOLE_GETCHAR => {
+                            // SAFETY: Safe because the exit_reason (which comes from the kernel) told us
+                            // which union field to use
                             let ch = unsafe { &mut run.__bindgen_anon_1.riscv_sbi.ret[..1] };
                             Ok(VcpuExit::SbiExt0_1ConsoleGetchar(ch))
                         }