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

David Woodhouse
2023-02-28T21:58:57Z

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On Tue, 2023-02-28 at 22:01 +0100, Thomas Gleixner wrote:
>
> 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 */
>
>         /*
>

So yeah, I'd frowned at the zero %edx there for a while (but evidently
not long enough to realise that even if it's right it should be %rdx).

But then figured it was an *offset*, that __per_cpu_offset[0] would be
zero too at least on first boot and probably forever, did a quick grep
for pcpu_hot and figured my brain hurt and Brian probably knew what he
was doing.

Empirically, yours doesn't boot and Brian's does. If I just fix it to
zero all of %rdx and then run (just up to the initial_gs patch) with
qemu -d in_asm ...

grep gdt_page System.map
ffffffff05d7a000 A init_per_cpu__gdt_page
ffffffff825cfeb0 r __ksymtab_gdt_page
ffffffff82811000 D gdt_page

0x060000a9: 48 c7 c0 b2 00 00 a2 movq $-0x5dffff4e, %rax
0x060000b0: ff e0 jmpq *%rax

----------------
IN:
0xffffffffa20000b2: 48 31 d2 xorq %rdx, %rdx
0xffffffffa20000b5: 48 8b 82 c0 74 d5 a3 movq -0x5c2a8b40(%rdx), %rax
0xffffffffa20000bc: 48 8b a0 58 14 00 00 movq 0x1458(%rax), %rsp
0xffffffffa20000c3: 48 83 ec 10 subq $0x10, %rsp
0xffffffffa20000c7: 66 c7 04 24 7f 00 movw $0x7f, (%rsp)
0xffffffffa20000cd: 48 8d 82 00 10 81 a3 leaq -0x5c7ef000(%rdx), %rax
0xffffffffa20000d4: 48 89 44 24 02 movq %rax, 2(%rsp)
0xffffffffa20000d9: 0f 01 14 24 lgdtq (%rsp)
0xffffffffa20000dd: 48 83 c4 10 addq $0x10, %rsp
0xffffffffa20000e1: 31 c0 xorl %eax, %eax
0xffffffffa20000e3: 8e d8 movl %eax, %ds

I cannot work out where the value -0x5c7ef000 comes from, but it
doesn't seem to be the 0xb000 you claimed, and my brain is hurting
again...

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