From:   Thomas Gleixner <tglx@linutronix.de>
To:     Usama Arif <usama.arif@bytedance.com>, dwmw2@infradead.org,
        arjan@linux.intel.com
Cc:     mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com,
        hpa@zytor.com, x86@kernel.org, pbonzini@redhat.com,
        paulmck@kernel.org, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org, rcu@vger.kernel.org, mimoja@mimoja.de,
        hewenliang4@huawei.com, thomas.lendacky@amd.com, seanjc@google.com,
        pmenzel@molgen.mpg.de, fam.zheng@bytedance.com,
        punit.agrawal@bytedance.com, simon.evans@bytedance.com,
        liangma@liangbit.com, Usama Arif <usama.arif@bytedance.com>
Subject: Re: [PATCH v6 06/11] x86/smpboot: Support parallel startup of
 secondary CPUs
In-Reply-To: <20230202215625.3248306-7-usama.arif@bytedance.com>
References: <20230202215625.3248306-1-usama.arif@bytedance.com>
 <20230202215625.3248306-7-usama.arif@bytedance.com>
Date:   Tue, 07 Feb 2023 01:07:45 +0100
Message-ID: <87v8kewd66.ffs@tglx>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

On Thu, Feb 02 2023 at 21:56, Usama Arif wrote:
> From: Thomas Gleixner <tglx@linutronix.de>
>
> To allow for parallel AP bringup, we need to avoid the use of global
> variables for passing information to the APs, as well as preventing them
> from all trying to use the same real-mode stack simultaneously.
>
> So, introduce a 'lock' field in struct trampoline_header to use as a
> simple bit-spinlock for the real-mode stack. That lock also protects
> the global variables initial_gs, initial_stack and early_gdt_descr,
> which can now be calculated...
>
> So how do we calculate those addresses? Well, they they can all be found
> from the per_cpu data for this CPU. Simples! Except... how does it know
> what its CPU# is? OK, we export the cpuid_to_apicid[] array and it can
> search it to find its APIC ID in there.
>
> But now you whine at me that it doesn't even know its APIC ID? Well, if
> it's a relatively modern CPU then the APIC ID is in CPUID leaf 0x0B so
> we can use that. Otherwise... erm... OK, otherwise it can't have parallel
> CPU bringup for now. We'll still use a global variable for those CPUs and
> bring them up one at a time.
>
> So add a global 'smpboot_control' field which either contains the APIC
> ID, or a flag indicating that it can be found in CPUID.
>
> This adds the 'do_parallel_bringup' flag in preparation but doesn't
> actually enable parallel bringup yet.
>
> [ dwmw2: Minor tweaks, write a commit message ]
> [ seanc: Fix stray override of initial_gs in common_cpu_up() ]
> Not-signed-off-by: Thomas Gleixner <tglx@linutronix.de>

I'm happy to add my SOB if someone actually writes a coherent changelog
which complies with the documented rules :)