Re: [PATCH v7 6/9] x86/smpboot: Support parallel startup of secondary CPUs

[David Woodhouse <dwmw2@infradead.org>]

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On Wed, 2023-02-08 at 14:13 +0100, Thomas Gleixner wrote:
> On Wed, Feb 08 2023 at 00:09, Brian Gerst wrote:
> > On Tue, Feb 7, 2023 at 6:10 PM Usama Arif <usama.arif@bytedance.com> wrote:
> > > Introduce a global variable 'smpboot_control' indicating to the AP how
> > > it should find its APIC ID. For a serialized bringup, the APIC ID is
> > > explicitly passed in the low bits of smpboot_control, while for parallel
> > > mode there are flags directing the AP to find its APIC ID in CPUID leaf
> > > 0x0b (for X2APIC mode) or CPUID leaf 0x01 where 8 bits are sufficient.
> > 
> > For the serialized bringup case, it would be simpler to just put the
> > cpu number in the lower bits instead of the APIC id, skipping the
> > lookup.
> 
> Yes and no. It can be done, but I rather prefer the consistency of the
> data and the mechanism. The "overhead" or "win" is not even measurable.


Yeah. I might do this much though, just to track CPU# simply in %ecx:

--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -285,17 +285,15 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify,
SYM_L_GLOBAL)
        leaq    cpuid_to_apicid(%rip), %rbx
 
 .Lfind_cpunr:
-       cmpl    (%rbx), %edx
+       cmpl    (%rbx,%rcx,4), %edx
        jz      .Linit_cpu_data
-       addq    $4, %rbx
-       addq    $8, %rcx
+       inc     %rcx
        jmp     .Lfind_cpunr
 
 .Linit_cpu_data:
-       /* Get the per cpu offset */
+       /* Get the per cpu offset for the given CPU# which is in ECX */
        leaq    __per_cpu_offset(%rip), %rbx
-       addq    %rcx, %rbx
-       movq    (%rbx), %rbx
+       movq    (%rbx,%rcx,8), %rbx
        /* Save it for GS BASE setup */
        movq    %rbx, initial_gs(%rip)
 


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