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Date:   Wed, 24 May 2023 23:48:18 +0300
From:   "Kirill A. Shutemov" <kirill@shutemov.name>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     LKML <linux-kernel@vger.kernel.org>, x86@kernel.org,
        David Woodhouse <dwmw2@infradead.org>,
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        Dave Hansen <dave.hansen@linux.intel.com>
Subject: Re: [patch v3 31/36] x86/apic: Provide cpu_primary_thread mask
Message-ID: <20230524204818.3tjlwah2euncxzmh@box.shutemov.name>
References: <20230508181633.089804905@linutronix.de>
 <20230508185218.962208640@linutronix.de>
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On Mon, May 08, 2023 at 09:44:17PM +0200, Thomas Gleixner wrote:
> From: Thomas Gleixner <tglx@linutronix.de>
> 
> Make the primary thread tracking CPU mask based in preparation for simpler
> handling of parallel bootup.
> 
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Tested-by: Michael Kelley <mikelley@microsoft.com>
> 
> 
> ---
>  arch/x86/include/asm/apic.h     |    2 --
>  arch/x86/include/asm/topology.h |   19 +++++++++++++++----
>  arch/x86/kernel/apic/apic.c     |   20 +++++++++-----------
>  arch/x86/kernel/smpboot.c       |   12 +++---------
>  4 files changed, 27 insertions(+), 26 deletions(-)
> ---
> 

...

> @@ -2386,20 +2386,16 @@ bool arch_match_cpu_phys_id(int cpu, u64
>  }
>  
>  #ifdef CONFIG_SMP
> -/**
> - * apic_id_is_primary_thread - Check whether APIC ID belongs to a primary thread
> - * @apicid: APIC ID to check
> - */
> -bool apic_id_is_primary_thread(unsigned int apicid)
> +static void cpu_mark_primary_thread(unsigned int cpu, unsigned int apicid)
>  {
> -	u32 mask;
> -
> -	if (smp_num_siblings == 1)
> -		return true;
>  	/* Isolate the SMT bit(s) in the APICID and check for 0 */
> -	mask = (1U << (fls(smp_num_siblings) - 1)) - 1;
> -	return !(apicid & mask);
> +	u32 mask = (1U << (fls(smp_num_siblings) - 1)) - 1;
> +
> +	if (smp_num_siblings == 1 || !(apicid & mask))
> +		cpumask_set_cpu(cpu, &__cpu_primary_thread_mask);
>  }
> +#else
> +static inline void cpu_mark_primary_thread(unsigned int cpu, unsigned int apicid) { }
>  #endif
>  
>  /*

This patch causes boot regression on TDX guest. The guest crashes on SMP
bring up.

The change makes use of smp_num_siblings earlier than before: the mask get
constructed in acpi_boot_init() codepath. Later on smp_num_siblings gets
updated in detect_ht().

In my setup with 16 vCPUs, smp_num_siblings is 16 before detect_ht() and
set to 1 in detect_ht().

-- 
  Kiryl Shutsemau / Kirill A. Shutemov