linux-kernel - Re: [PATCH v6 06/11] x86/smpboot: Support parallel startup of secondary CPUs

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Date:   Thu, 02 Feb 2023 22:30:30 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Usama Arif <usama.arif@...edance.com>, tglx@...utronix.de,
        arjan@...ux.intel.com
Cc:     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 v6 06/11] x86/smpboot: Support parallel startup of
 secondary CPUs

On Thu, 2023-02-02 at 21:56 +0000, Usama Arif wrote:
> @@ -1515,6 +1530,17 @@ void __init native_smp_prepare_cpus(unsigned int max_cpus)
>  
>         speculative_store_bypass_ht_init();
>  
> +       /*
> +        * We can do 64-bit AP bringup in parallel if the CPU reports its
> +        * APIC ID in CPUID leaf 0x0B. Otherwise it's too hard. And not
> +        * for SEV-ES guests because they can't use CPUID that early.
> +        * Also, some AMD CPUs crash when doing parallel cpu bringup, disable
> +        * it for all AMD CPUs to be on the safe side.
> +        */
> +       if (IS_ENABLED(CONFIG_X86_32) || boot_cpu_data.cpuid_level < 0x0B ||
> +           cc_platform_has(CC_ATTR_GUEST_STATE_ENCRYPT))
> +               do_parallel_bringup = false;
> +
>         snp_set_wakeup_secondary_cpu();
>  }

Oops, I thought I'd removed those two lines about AMD in the comment
when I removed the actual code that did so. Turns out we remove those
lines again in the *next* patch instead. 

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