Re: [PATCH v12 07/11] x86/smpboot: Remove early_gdt_descr on 64-bit

Thomas Gleixner
2023-02-28T21:01:11Z

On Sun, Feb 26 2023 at 11:07, Usama Arif wrote:
> @@ -265,7 +265,12 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
> * addresses where we're currently running on. We have to do that here
> * because in 32bit we couldn't load a 64bit linear address.
> */
> - lgdt early_gdt_descr(%rip)
> + subq $16, %rsp
> + movw $(GDT_SIZE-1), (%rsp)
> + leaq gdt_page(%rdx), %rax

Even on !SMP gdt_page is in the 0...__per_cpu_end range. Which means
that on !SMP this results in:

leaq 0xb000(%rdx),%rax

and RDX is 0. That's not really a valid GDT pointer, right?

> + movq %rax, 2(%rsp)
> + lgdt (%rsp)

and obviously that's equally broken for the task stack part:

> movq pcpu_hot + X86_current_task(%rdx), %rax

This needs:

--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -239,7 +239,7 @@ SYM_INNER_LABEL(secondary_startup_64_no_
/* Get the per cpu offset for the given CPU# which is in ECX */
movq __per_cpu_offset(,%rcx,8), %rdx
#else
- xorl %edx, %edx
+ leaq INIT_PER_CPU_VAR(fixed_percpu_data)(%rip), %rdx
#endif /* CONFIG_SMP */

/*

in the initial_stack patch, which then allows to remove this hunk in the
initial_gs patch:

@@ -286,9 +286,6 @@ SYM_INNER_LABEL(secondary_startup_64_no_
* the per cpu areas are set up.
*/
movl $MSR_GS_BASE,%ecx
-#ifndef CONFIG_SMP
- leaq INIT_PER_CPU_VAR(fixed_percpu_data)(%rip), %rdx
-#endif
movl %edx, %eax
shrq $32, %rdx
wrmsr

Maybe we should enforce CONFIG_SMP=y first :)

Thanks,

tglx