From:   Thomas Gleixner <tglx@linutronix.de>
To:     Brian Gerst <brgerst@gmail.com>,
        Usama Arif <usama.arif@bytedance.com>
Cc:     dwmw2@infradead.org, kim.phillips@amd.com, arjan@linux.intel.com,
        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
Subject: Re: [PATCH v7 6/9] x86/smpboot: Support parallel startup of
 secondary CPUs
In-Reply-To: <CAMzpN2iejCnBeBdC6+92fUL2k8ZdAq_jEgXX+RSoGMhRZ0UBSA@mail.gmail.com>
References: <20230207230436.2690891-1-usama.arif@bytedance.com>
 <20230207230436.2690891-7-usama.arif@bytedance.com>
 <CAMzpN2iejCnBeBdC6+92fUL2k8ZdAq_jEgXX+RSoGMhRZ0UBSA@mail.gmail.com>
Date:   Wed, 08 Feb 2023 14:13:18 +0100
Message-ID: <87k00s2tch.ffs@tglx>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

On Wed, Feb 08 2023 at 00:09, Brian Gerst wrote:
> On Tue, Feb 7, 2023 at 6:10 PM Usama Arif <usama.arif@bytedance.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.

Thanks,

        tglx