linux-kernel - Re: [External] Re: [PATCH v8 8/9] x86/mtrr: Avoid repeated save of MTRRs on boot-time CPU bringup

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Message-ID: <704abe334aa3166b579e1ba00369c977b1ea4d24.camel@infradead.org>
Date:   Thu, 09 Feb 2023 20:37:13 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Usama Arif <usama.arif@...edance.com>,
        Thomas Gleixner <tglx@...utronix.de>, kim.phillips@....com
Cc:     arjan@...ux.intel.com, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, x86@...nel.org,
        pbonzini@...hat.com, paulmck@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        rcu@...r.kernel.org, mimoja@...oja.de, hewenliang4@...wei.com,
        thomas.lendacky@....com, seanjc@...gle.com, pmenzel@...gen.mpg.de,
        fam.zheng@...edance.com, punit.agrawal@...edance.com,
        simon.evans@...edance.com, liangma@...ngbit.com
Subject: Re: [External] Re: [PATCH v8 8/9] x86/mtrr: Avoid repeated save of
 MTRRs on boot-time CPU bringup

On Thu, 2023-02-09 at 20:32 +0000, Usama Arif wrote:
> 
> I will let David confirm if this is correct and why he did it, but this 
> is what I thought while reviewing before posting v4:
> 
> - At initial boot (system_state < SYSTEM_RUNNING), when mtrr_save_state 
> is called in do_cpu_up at roughly the same time so MTRR is going to be 
> 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.
> 
> - When the system is running and you offline and then online a CPU, you 
> want to make sure that hotplugged CPU gets the current MTRR (which might 
> have changed since boot?), incase the MTRR has changed after the system 
> has been booted, you save the MTRR of the first online CPU. When the 
> hotplugged CPU runs its initialisation code, its fixed-range MTRRs will 
> be updated with the newly saved fixed-range MTRRs.
> 
> So mainly for hotplug, but will let David confirm.

Sounds about right.

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