On Tue, 2023-03-21 at 14:28 -0400, Brian Gerst wrote:
>
> > @@ -264,6 +318,14 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
> > lgdt (%rsp)
> > addq $16, %rsp
> >
> > + /* Drop the realmode protection. For the boot CPU the pointer is NULL! */
> > + movq trampoline_lock(%rip), %rax
> > + testq %rax, %rax
> > + jz .Lsetup_data_segments
> > + lock
> > + btrl $0, (%rax)
> > +
> > +.Lsetup_data_segments:
> > /* set up data segments */
> > xorl %eax,%eax
> > movl %eax,%ds
>
> This can still go earlier, right after "movq TASK_threadsp(%rax),
> %rsp". The GDT descriptor is placed on the idle thread stack, so it's
> safe to drop the lock before it.
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -335,6 +335,17 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
movq pcpu_hot + X86_current_task(%rdx), %rax
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
+ testq %rax, %rax
+ jz .Lsetup_gdt
+ lock
+ btrl $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
@@ -377,14 +388,6 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
shrq $32, %rdx
wrmsr
- /* Drop the realmode protection. For the boot CPU the pointer is NULL! */
- movq trampoline_lock(%rip), %rax
- testq %rax, %rax
- jz .Lsetup_idt
- lock
- btrl $0, (%rax)
-
-.Lsetup_idt:
/* Setup and Load IDT */
pushq %rsi
call early_setup_idt
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