Message-ID: <35264451eabdf967eb31069cb814e8a05ee3179b.camel@infradead.org>
Subject: Re: [PATCH v12 07/11] x86/smpboot: Remove early_gdt_descr on 64-bit
From:   David Woodhouse <dwmw2@infradead.org>
To:     Thomas Gleixner <tglx@linutronix.de>,
        Usama Arif <usama.arif@bytedance.com>, kim.phillips@amd.com,
        brgerst@gmail.com
Cc:     piotrgorski@cachyos.org, oleksandr@natalenko.name,
        arjan@linux.intel.com, mingo@redhat.com, bp@alien8.de,
        dave.hansen@linux.intel.com, hpa@zytor.com, x86@kernel.org,
        pbonzini@redhat.com, paulmck@kernel.org,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        rcu@vger.kernel.org, mimoja@mimoja.de, hewenliang4@huawei.com,
        thomas.lendacky@amd.com, seanjc@google.com, pmenzel@molgen.mpg.de,
        fam.zheng@bytedance.com, punit.agrawal@bytedance.com,
        simon.evans@bytedance.com, liangma@liangbit.com
Date:   Tue, 28 Feb 2023 21:57:50 +0000
In-Reply-To: <878rghmrn2.ffs@tglx>
References: <20230226110802.103134-1-usama.arif@bytedance.com>
         <20230226110802.103134-8-usama.arif@bytedance.com> <878rghmrn2.ffs@tglx>
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On Tue, 2023-02-28 at 22:01 +0100, Thomas Gleixner wrote:
>=20
> This needs:
>=20
> --- 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_
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0/* Get the per cpu offset=
 for the given CPU# which is in ECX */
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0movq=C2=A0=C2=A0=C2=A0=C2=
=A0__per_cpu_offset(,%rcx,8), %rdx
> =C2=A0#else
> -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0xorl=C2=A0=C2=A0=C2=A0=C2=A0%e=
dx, %edx
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0leaq=C2=A0=C2=A0=C2=A0=C2=A0IN=
IT_PER_CPU_VAR(fixed_percpu_data)(%rip), %rdx
> =C2=A0#endif /* CONFIG_SMP */
> =C2=A0
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0/*
>=20

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=20
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:=20
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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