linux-kernel - Re: [patch v3 31/36] x86/apic: Provide cpu_primary

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Date:   Fri, 26 May 2023 12:14:21 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        David Woodhouse <dwmw2@...radead.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Brian Gerst <brgerst@...il.com>,
        Arjan van de Veen <arjan@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Paul McKenney <paulmck@...nel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Sean Christopherson <seanjc@...gle.com>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        Paul Menzel <pmenzel@...gen.mpg.de>,
        "Guilherme G. Piccoli" <gpiccoli@...lia.com>,
        Piotr Gorski <lucjan.lucjanov@...il.com>,
        Usama Arif <usama.arif@...edance.com>,
        Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        xen-devel@...ts.xenproject.org,
        Russell King <linux@...linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>,
        linux-arm-kernel@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Guo Ren <guoren@...nel.org>,
        linux-csky@...r.kernel.org,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        linux-mips@...r.kernel.org,
        "James E.J. Bottomley" <James.Bottomley@...senPartnership.com>,
        Helge Deller <deller@....de>, linux-parisc@...r.kernel.org,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        linux-riscv@...ts.infradead.org,
        Mark Rutland <mark.rutland@....com>,
        Sabin Rapan <sabrapan@...zon.com>,
        "Michael Kelley (LINUX)" <mikelley@...rosoft.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [patch v3 31/36] x86/apic: Provide cpu_primary_thread mask

On Wed, May 24 2023 at 23:48, Kirill A. Shutemov wrote:
> On Mon, May 08, 2023 at 09:44:17PM +0200, Thomas Gleixner wrote:
>>  #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.

I rather call it a security feature: It makes TDX unbreakably secure.

> 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().

  early_init_intel(c)
    if (detect_extended_topology_early(c) < 0)
       detect_ht_early(c);

  acpi_boot_init()
    ....

  identify_boot_cpu(c)
    detect_ht(c);

Aaargh. That whole CPU identification code is a complete horrorshow.

I'll have a look....