Re: [patch v3 34/36] x86/smpboot: Implement a bit spinlock to protect the realmode stack

Peter Zijlstra
2023-05-09T13:15:44Z

On Mon, May 08, 2023 at 09:44:22PM +0200, Thomas Gleixner wrote:

> @@ -252,6 +252,17 @@ SYM_INNER_LABEL(secondary_startup_64_no_
> movq TASK_threadsp(%rax), %rsp
>
> /*
> + * Now that this CPU is running on its own stack, drop the realmode
> + * protection. For the boot CPU the pointer is NULL!
> + */
> + movq trampoline_lock(%rip), %rax
movl $0, (%rax)

> +.Lsetup_gdt:
> + /*
> * We must switch to a new descriptor in kernel space for the GDT
> * because soon the kernel won't have access anymore to the userspace
> * addresses where we're currently running on. We have to do that here

> --- a/arch/x86/realmode/rm/trampoline_64.S
> +++ b/arch/x86/realmode/rm/trampoline_64.S
> @@ -37,6 +37,24 @@
> .text
> .code16
>
> +.macro LOAD_REALMODE_ESP
> + /*
> + * Make sure only one CPU fiddles with the realmode stack
> + */
> +.Llock_rm\@:
> + btl $0, tr_lock
> + jnc 2f
> + pause
> + jmp .Llock_rm\@
> +2:
> + lock
> + btsl $0, tr_lock
> + jc .Llock_rm\@

Do we really care about performance here; or should we pick the simpler
form? Also, 'lock' is a prefix, not an instruction.

.Llock_rm\@:
lock btsl $0, tr_lock;
jnc 2f
pause
jmp .Llock_rm\@
2:

> +
> + # Setup stack
> + movl $rm_stack_end, %esp
> +.endm
> +
> .balign PAGE_SIZE