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

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Message-ID: <c35b903f44c54510b71dca3bc2ecb6ee257cffba.camel@infradead.org>
Date:   Wed, 08 Feb 2023 14:41:51 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Brian Gerst <brgerst@...il.com>,
        Usama Arif <usama.arif@...edance.com>
Cc:     kim.phillips@....com, 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 v7 6/9] x86/smpboot: Support parallel startup of
 secondary CPUs

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@...edance.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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