Message-ID: <704abe334aa3166b579e1ba00369c977b1ea4d24.camel@infradead.org>
Subject: Re: [External] Re: [PATCH v8 8/9] x86/mtrr: Avoid repeated save of
 MTRRs on boot-time CPU bringup
From:   David Woodhouse <dwmw2@infradead.org>
To:     Usama Arif <usama.arif@bytedance.com>,
        Thomas Gleixner <tglx@linutronix.de>, kim.phillips@amd.com
Cc:     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
Date:   Thu, 09 Feb 2023 20:37:13 +0000
In-Reply-To: <9b6bca9c-7189-a2d5-8c0a-f55c24f54b62@bytedance.com>
References: <20230209154156.266385-1-usama.arif@bytedance.com>
         <20230209154156.266385-9-usama.arif@bytedance.com> <87mt5m1yiz.ffs@tglx>
         <9b6bca9c-7189-a2d5-8c0a-f55c24f54b62@bytedance.com>
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On Thu, 2023-02-09 at 20:32 +0000, Usama Arif wrote:
>=20
> I will let David confirm if this is correct and why he did it, but this=
=20
> is what I thought while reviewing before posting v4:
>=20
> - At initial boot (system_state < SYSTEM_RUNNING), when mtrr_save_state=
=20
> is called in do_cpu_up at roughly the same time so MTRR is going to be=
=20
> the same, we can just save it once and then reuse for other secondary
> cores as it wouldn't have changed for the rest of the do_cpu_up calls.
>=20
> - When the system is running and you offline and then online a CPU, you=
=20
> want to make sure that hotplugged CPU gets the current MTRR (which might=
=20
> have changed since boot?), incase the MTRR has changed after the system=
=20
> has been booted, you save the MTRR of the first online CPU. When the=20
> hotplugged CPU runs its initialisation code, its fixed-range MTRRs will=
=20
> be updated with the newly saved fixed-range MTRRs.
>=20
> So mainly for hotplug, but will let David confirm.

Sounds about right.

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