linux-kernel - Re: [PATCH v12 07/11] x86/smpboot: Remove early_gdt

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Message-ID: <35264451eabdf967eb31069cb814e8a05ee3179b.camel@infradead.org>
Date:   Tue, 28 Feb 2023 21:57:50 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Usama Arif <usama.arif@...edance.com>, kim.phillips@....com,
        brgerst@...il.com
Cc:     piotrgorski@...hyos.org, oleksandr@...alenko.name,
        arjan@...ux.intel.com, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, x86@...nel.org,
        pbonzini@...hat.com, paulmck@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        rcu@...r.kernel.org, mimoja@...oja.de, hewenliang4@...wei.com,
        thomas.lendacky@....com, seanjc@...gle.com, pmenzel@...gen.mpg.de,
        fam.zheng@...edance.com, punit.agrawal@...edance.com,
        simon.evans@...edance.com, liangma@...ngbit.com
Subject: Re: [PATCH v12 07/11] x86/smpboot: Remove early_gdt_descr on 64-bit

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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